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Posted: 4/9/2013 9:57:36 AM EDT
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I just finished my lr308 build.
If I'm lucky I can get 3 shots off without a malfunction. The bolt doesn't lock back after the last round in the mag. It locks back fine using the charging handle. It's a 22" 308 win with rifle length gas from fulton, i got a dpms stock assembly for the lr308 which included a 308 buffer and spring. the gas key is staked, nice fresh gas rings and bolt, the bolt travels smoothly when the gas tube and gas key interface, applied clp liberally to bolt, carrier and buffer spring. I thought the gas block might not have been lined up at first so i tried to align it better and used a piece of aquarium tubing to test the airflow and it seemed to eject the spent cases more frequently but still won't lock back. Where do I go from here? I put in an order for a new gas block partly because the one I have doesn't fit under the rail very well and partly because it might help... |
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You are using both a DMPS chambered barrel and DPMS bolt, and have not slipped an armilite bolt in this mix?
The Armalite barrel has to be used with an Armalte bolt (and visa veras) since the two are not the same parts, will not head space correctly if you don't match them up. Also, go ahead and list the rest of the parts as well (including FCG) since again, you have the Armalite dimension part system, and the DPMS dimension part system, requiring specific parts as well. |
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no armalite dimension parts in the mix.
from tip to tail yhm muzzle break fulton armory 22" 1:12 stainless with rifle gas 308 match chambered fulton arms gas block i think the gas tube is dpms troy trx 13.8" rail fulton arms titan slick slide flat top upper fulton arms bolt carrier jp enhanced bolt assembly dpms titanium nitride coated gas key dpms bolt completion pack (firing pin, firing pin retainer, cam pin) cmmg billet lower dpms lpk dpms lr308 carbine buttstock assembly swapped stock with a magpul acsl bcm charging handle primary arms scope 20rd pmags fresh from wrapper have tried pmc 147 grain and some surplus i think portugese? I'd have to check the box at home. and here's a pic for fun 
doesn't an armalite bolt have a different number of lugs it wouldn't even lock up or fire if i had that mismatched. |
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The port ammo is going to run on the hotter side (love the ammo through Fal's and HKs), so might want to thing about trying some factory 308 as a quick test.
On the B/C with a plated key, make sure that the B/C slips in the upper without any binding. NO, the DPMS and Armalite B/C and barrel types look the same, but they have to go with the same type barrel/bolt to head space correctly. On the charging handle pull when you go to lock the bolt back by CH pull, is the charging handle pull for the last inch harder tension than the rest of the pull. Also, did you clean the chamber with a chamber brush and CLP before firing the weapon??? The barrels come with storage grease to prevent rusting, and you have to get all that out before the rifle is fired. Short of the above, go back through the gas system and check for leaks, such as the the gas block to barrel, gas block to gas tube, and even pressurize the B/C with the bolt held in to make sure that you don't have a leak between the key to carrier. Also touch base with Futon if you bought the barrel and Bolt from them since they should have check head space of the two. Also confirm with them what size the gas port should be, so you can double check that as well. |
the pmc seemed a little worse plus the cases crushed a lot easier when there was a malfunction destroying my brass
the bolt is from jp they don't make an armalite one ... never checked headspace I suppose I can hit up the htf and the lgs to see if someones got the gauges. You think that could make it shortstroke? The brass come out looking pretty good not sooty no hard extractor or ejector marks no evidence of imminent case head separation. i am pretty sure i neglected to clean the barrel as well, it did not seem greasy but that'd make sense. I'll do the other checks when i get home |
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so i tried blowing through the key with the bolt pushed in and the vents covered on the bc and i could hear a little air from somewhere,
so i covered it in soapy water and i could see bubbles coming out at the front of the key where it meets the carrier. I tried to tighten it more and was able to budge the screws a fraction of a milimeter but tried the soap test again and still had little bubbles. Took the key off and here is what it looks like underneath. 
i wiped it off before taking the pic you can see where the gas was escaping towards the front of the key. here is the carrier 
that's just a shiny area on the carrier the finish is still there but you can see where the key contacts it. It looks like we don't have a nice flush mate between the key and the carrier. some sealant? or i maybe try and scrub down the phosphate in the corners a bit? |
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Time for lapping.
Start with the bottom of the key on a straight lapping plate, some 400 sandpaper or lapping compound on the plate. You want the bottom of the key flat/true, so don't worry about removing the finish on the bottom of the key. Once you have the bottom of the key flat and square, put some lapping compound on the top of he carrier, and now lap the key into the carrier with forward/rearward strokes of the key against the top of the carrier. Watch the lapping marking to see when the top of the carrier lapping scratches are consistent all the way across. At this point, flush all the lapping compound off with acetone, put a thin layer of Red loctite across the top of the carrier surface where the key is going to seat (loctite being used as a gasket material, and not as loctite to loctite the bolts in place) install/torque the key bolts to 37in lbs, then stake the top of the key to peen metal from the key into the side spines of the key bolts. Note: Do not strike the top of the bolts with the punch at all to try to peen the bolt tops. Its the metal from the key that has to be flowed into the side spines of the bolt instead. The top and bottom B/C have been staked correctly, while middle staking is not correct. 
Also, once done, make sure to go back and double check the alignment of the gas tube to the key, and if needed tweak the gas tube over the barrel to correct the alignment (use the carrier without the bolt installed in the upper receiver to check the alignment). |
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Quoted:
Dano, This is one of the clearest explanations of anything that I have ever read. I hope I never have to use it but thanks. No worries, and if you do every have to lap a key it, it goes pretty quick. 99% of the time, both surfaces are pretty true to begin with, and it just the protruding burs that have to be knocked down instead. |
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got the bottom of the key nice and flat.
It definitely had some unevenness I just laid fine emery cloth on my glass table and worked the key on it until it made even marks across the entire surface. took all of the finish off the bottom. I measured the thickness before and after and came up with 0.237" before and after here is a pic of the key 
had to go to o'reily auto parts for lapping compound, they only had permatex red thread locker which i don't think will be suitable for this application it's optimum temperature range is -65 to 300 while red loctite goes up to 550. Here is the lapping compound and thread locker. 
sorry for the blurry phone photo. can i use this thread locker or should i really use real red loctite? |
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Go hit the wife/girlfriend up for some fingernail polish remover, and your off to the races.
The acetone is just so the loctite gets a good bond to the bottom of the key and carrier surfaces so it creates a bonded gasket. The thread locker you have might work, but better off with real Loctite instead. The concept her is that you are making a micro gasket in the range of less than .001" at most in the void areas, and the higher temp it takes to break down the locker, the better off you are. And no on the gel type, you want the standard loctite that will flow easier. Also, some acetone on a Q tip does a great job on cleaning up the excess loctite that flows out onto the carrier and key mate up after you have the key bolts torqued down to 37 in lbs and the key staked, and give the key a quick lower pressure shot of air down it with the bolt out to flow out any loctite that has collected in the gas passage channel while the loctite is still fluid (so you don't end up with a layer of lock tight across the gas passage channel that is going to end up in the gas chamber latter). As for set up time, give the loctite a good day to set up before you put the B/C into service. The allows the loctite to set up solid to become a great micro gasket to fill any voids between the two and prevent blow-past of any kind. |
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Thanks Dano,
You've been a great help. Hopefully this makes enough of a difference it was not that great of a leak. Will head to the blacksmiths house and steal his loctite and make him help me re-stake the key. Any suggestion for what to cover the raw metal on the carrier behind the key with? |
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After you have reinstalled the key,
Birch-wood/Casey "Super Black" will Cold blue the raw metal pretty much back to the same color at the rest of the parkerized carrier surface (dark black), and like the parkerizing, the cold blued surface will help to retain CLP to prevent the metal from rusting. Just get the small raw metal surface oil free with Acetone, a little of the super black on a Q tip,and a quick rub down on the bare metal will blacken it instantly. Don't worry about washing the metal off with water, just hit the new blackened surface with CLP and a slight rub down. Note: On the super black and larger areas, if you go overboard trying to blacken it too much ( chemical etch the surface too much), the surface will a kind of scale (start to look like blotched heat case bluing instead), and here, before you hit the metal with oil to neutralize the acid, hit the surface with SS wool to knock the scaling off, then with the super blue still on the Q tip about half strength now, go over the metal again to re-darken it back up. Also, if you have to pick up super black (read the smith that you are going to do not have some), might want to pick up a bottle of aluminum black as well. Same with the acetone first to degrease, and great for touching up small scratches on the aluminum black anodized parts. Note, it does not re-anodize the parts, just etches the raw surface of the aluminum black instead. |
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No love, it ejected everything but no bolt hold back and wouldn't pick up the next round.
The only other evidence I can find of escaping gas would be where the key and gas tube mate, kinda looks like gas blowing forward and leaving residue on the inside of the upper around the gas tube. I think it would do that anyway normally after the key and tube separate. Gas block still back ordered ... |
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it's the one that came with this kit http://www.midwayusa.com/product/238414/dpms-buttstock-assembly-6-position-collapsible-lr-308-carbine-synthetic-black
it's shorter than a regular ar15 carbine buffer. The bolt can freely travel and lock back it's just not getting enough gas or the spring is too strong... |
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Ok, with a enough rounds fired, lets pull the gas block and check the burn mark at the barrel gas port.
If you have the gas block on correctly, the burn mark is going to be dead center around the barrel gas port. Also, with the gas block off, pull the back set screw, and look up through the set screw channel to confirm that the side hole of the gas channel it aligned with the block gas passage. If needed, you can drill up through the gas block threaded hole to enlarge the gas tube side hole. As for the gas block, donor allen wrench in the set screws, heat the wrench up to break the old loctite bond, and then use a new allen wrench to loosen the set screws. While you have the gas block off, double check the size of the barrel gas port. When you put the gas bock on, red loctite on the set screws again, and don't forget to check the alignment of the key to gas tube with the carrier alone without the bolt. About all that I have left to offer is either the head space it too tight, the gas port is too small, the key is binding in the upper receiver slot, or the hammer is wedging on the carrier on the way back. |
pulled rail off more signs of leakage between gas tube and gas block.
 
pulled gb, it was a little too far forward as you can see by the burn mark. More notably look at all the crap on the barrel under there we used a little high temp wheel bearing grease to get the gas block on there so not sure if that's what we're seeing or if it's more gas leakage i think the latter. Port measures .098" (I have pretty inexpensive frankford Arsenal digital calipers)  
Inside the gas block, the hole in the gas tube is bigger than the hole in the gas block and only the far side of the gas tube is visible, couldn't really get a pic but here's moar crap 
also did a little check on the chamber, the rounds drop in easily and just a light tap of my palm on the rear of the upper lets the round fall out. |
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Well, you found your other major gas leak at least.
This one may or may not be an easy fix, and need a good photo on the inside ID of the gas block to see if the problem is just a high ridge at the gas passage area that is not allowing the block to seat flush with the top of the barrel, or if the problem is worse. The bottom set screws will hold the top of the block ID tightly to the top of the barrel to prevent the ghastly leak, but only if the surfaces are cleanly machined to begin with. And no, although you want to use loctite on the set screws to hold them in place, you do not want to use loctite between the barrel OD to gas block ID. The problem here is even if you use heat to break down the loctite bond, you still have a build up of material that will not allow the two to separate down the line (at least not without destroying the ID surface on the block). Also, may as well pull the gas tube out of the block and check the fit of these two as well. If the Gas tube is loose in the block tube channel, that at least that can be loctite in to solve the leak there. |
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Well i'm glad i put a new gas block on order before even starting this thread. I was kinda unhappy with this one from the start. It only has one set screw. And i don't know if you can tell but we had to modify the outer dimensions to get it to fit under the rail. Maybe we screwed it up then but it was a tight wierd fit on the barrel before that. There aren't any large ridges on the id of the gas block but the machine marks aren't very smooth and uniform
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Quoted:
Well i'm glad i put a new gas block on order before even starting this thread. I was kinda unhappy with this one from the start. It only has one set screw. And i don't know if you can tell but we had to modify the outer dimensions to get it to fit under the rail. Maybe we screwed it up then but it was a tight wierd fit on the barrel before that. There aren't any large ridges on the id of the gas block but the machine marks aren't very smooth and uniform I see two hole on the bottom, so tapping the second hole for a second set screw is a walk in the park. How deep are the inside ID channel grooves at the gas passage side, and how did you go about reducing the block OD for the float tube? If you have a lathe handy, then the clean up of the ID should be a walk in the part (but having said that, so would just making a new gas block as well). |
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I don't have any taps to tap the second hole.
There really aren't much of any grooves on the inside of the gas block not that i can really feel or measure, it just kinda looks like it was stuck over something spinning kind of sloppily to polish it out... No lathe we just ground a couple of the rounded surfaces down flat, If you look at the pic of it you can see the four corners that were rounded are angles. |
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You can pick up a set screw, and the needed size pilot drill bit and tap at home depot for under $10.
If the set screw in play will not fit into the other hole that was drilled(but not tapped), then just take the set screw you have, go to the hardware section to use the thread test plate to determine the size of the set screw you have now, grab another set screw out of the drawer cabinet that they keep them in to match the old set screw, then go to the tool section to grab a tap with drill bit in the small blister pack. If the set screw you have now will drop through the untapped hole, then pick up two of the next size up set screws, and the pilot drill bit and tap for them. Granted that a tapping wrench would be wise to pick up as well, but you can just chuck the tap up in a drill chuck, and spin the gas block on the tap to thread it. Start with the drill bit in the pack to make sure that the pilot channel was drilled correctly for the tap to begin with, and then thread the channel that should have been threaded. If you are going with two new larger set screws, then pilot bit drill out both channels, and tap them both. Also, use some Oil on the tap and channel as you are threading. Start the tap in a few turns, than as you start to feel increased resistance, back the tap off about a half turn to break the coiling metal coming off the tap to clear the tap relief channels, take the tap in another few threads until the tension starts to build again, back the tap off a half turn again, and so on and so on until you have the channel threaded. If you keep clearing the tap (breaking the coiling metal to small pieces) by backing it off a half turn or more as the tension build up (metal being cut off loading up the tap relief channels), you will not break a tap. But the question comes back to if the gas passage side of the block ID can be clean up so it not leaking when mated to the top of the barrel, even with the block correctly instead with the two loctite'd set screws? |
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Well that sounds fun enough while i wait on troy... starting to think i should see if someone else has something in stock.
Never did tap something before, i actually tried to put the screw in the untapped hole at first and was mad they only gave me one screw. It seems to be the right size where i could just tap it with the same sized screw. I don't think tapping it alone will work, i measured the id of the gb got between .935 in a couple spots that looked like they were rubbing, this is also where the powder residue seems to end. The rest was .938. My measuring may be less than ideal So i think the answer to your question could be moar fine emery cloth on a dowel? |
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Quoted:
So i think the answer to your question could be more fine emery cloth on a dowel? NO, the ID needs to be bored to a parallel it .938 true. Just trying to use the emery cloth on a Block ID will just increase the larger surfaces at the same time that you are increasing the smaller diameter as well, with you ending up with just a worst problem. As stated, if your can get the passage side true and straight, then the bottom pressure of the set screws will seal off the passage to gas block port. Hate to say it, but short of a lathe, or mill with a boring head to true the block ID to .938, then the only other thing that I could see having a possibility of working is a brake cylinder honing tool to try to true the ID instead. Simply, your at $10 to tap the second channel for a set screw, then another $30 if you get a honing tool, and still a crap shoot at correcting the gas block. At this point, either time to buy another gas block, or have someone make you a custom one instead. |
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GAH ARGH!!!
So i got a 7/8 dowell wrapped in fine emery cloth and smoothed it out, there was also a little ding in the barrel i polished off. Loaded on round and... the bolt locked back! Yay! Load three first one fired and cycled the next round in, second round fires and then i get the third round trying to feed in to a half extracted round, shake empty case out last round fires and bolt holds open. Tryed some more and was getting consistent bolt lock back and even managed to fire off 6 whole rounds in a row. So. I am pretty sure i got the gas system going but need an oring under the extractor or something. The most telling sign was i got a bolt held open and a half extracted round still in chamber. It didn't rip the rim off or leave a huge mark from the extractor, there was a pretty good size ejector swipe on the case head... |
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Chamber is nice and pretty looking, i had a ruff chamber before, my first trouble shooting thread, the emery cloth and dowell fixed that too lol. Although i ended up just putting an 11" barrel on it instead.
This one the rounds aren't stuck in the chamber like that, they are most of the way out of the chamber but seem to be popping off the bolt before they fully extract only sometimes. |
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Obo2, if the bolt is locking back all the time, then you have the gas leak problem solved at least.
As for the extractor, an added O ring may be the step in the right direction, but double check the grabbing claws in the extractor. Here, you are looking for claw edge tip burs and end of extractor rim relief channel that may not be allowing the entire surface of the extractor to get a good/full grasp on the case rim. If round, just use a small jewelers file to remove the burs, while leaving the edge of the grabbing claw sharp. As for extractor swipe, double check the extractor tension to make sure that the spring is not coiling binding up under full extractor depression, the bolt face at the extractor channel does not have a high ridge burs, and the the face of the extractor edges have been slightly radius instead of a sharp edges. |
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Quoted:
snip It didn't rip the rim off or leave a huge mark from the extractor, there was a pretty good size ejector swipe on the case head... I think ziarre rifle man was actually on to something there was a little tiny piece of metal flash right up in the throat/neck area that didn't come out when i scrubbed the chamber before and never noticed. it came out with a chamber brush and now when i put a round in it falls out instead of having to tap it on my hand... I'll pick up an oring too |
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Quoted:
Still having ejection issues... Maybe swap some of the buffer weights out with tungsten? Dropped case off the bolt face still on the pull, or stove piping problems instead. If you installed the O ring/the extractor spring is strong, and the extractor is still slipping off the case with bending the rim to hell, then you have a extractor problems plan and simple. Granted that adding buffer weight will slow down the unlock, but could lead you back to the same problem as before, with the bolt not coming back far enough/not full stroking. |
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no stovepiping, rims are not bent to hell either. cases are most of way extracted, end up with empty case partway in barrel extension with new round trying to feed under it. I have a different extractor i could put on there... Try the new extractor, and also do a few bolt lock backs with only a single round in the mag charged and fired to confirm that the bolt is dropping the case, and it not a matter of maybe a bad mag that is angling the top round in the mag tip upwards and what is knocking the spent case off the bolt face instead. With a bolt lock back, the spent case should be ejected a good 10' to 15' out of the ejection port. Also, confirm that the ejector is moving freely in the bolt face channel as well (just of lot of spring tension to it). Just hook a spent case off the extractor, and cam it in and out a few times to check for no binding. We are thinking that the extractor is dropping the case on the way back, but it could be that the bolt is never letting go of the spent case instead, and just feeding it right back into the chamber on the way forward instead. |
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Quoted:
at least twice the bolt locked back and the empty was just loosely sitting in the chamber/barrel ext. ejector does not seem to be binding at all i can cam a case in and let it go and fling it across the room. Ejector is fine, so now is coming down to the extractor having a good retention of the spent case rim under residual bore pressure that will semi bind the spent case to the chamber walls during a live fire pull, and as stated, if the ejector was doing it job correctly to retain the spent case to the bolt face on the pulled ( such as in the case of over function), then the spent case rim should be bent to hell or sheared off instead. Simply here, as you where checking the ejector, the rim OS should have fit the bolt recess channel on the tighter side, and the claw of the extractor should have seated in a manner it got the most over lap on the rim as possible. Also, with the spent case back out, double check the bottom channel wall of the rim relief groove on the extractor. It should be flush with the bolt recessed face, and not below or proud of the recess of the bolt face surface. Lastly, double check to confirm that the face of the bolt is not retracting back past the back of the ejection port on the rear stroke. When this happens, the spent case of the pivot off the bolt at back of stroke can strike the back of the ejection port window to deflect the case right back into the action. To test this, pull the charging handle all the way back and look. If the face of the bolt is retracting back past the ejection port window back edge, then pull the buffer and buffer spring and start dropping quarters down to the tube to shim out the over depth of the receiver extension void not needed. As for a more permanent solution to this problem, then the front of the receiver extension needs to be face to set it back so it can be threaded farther into the lower receiver (while indexing the tube correctly for the butt stock while the end of the tube kisses the center post of the buffer retainer). P.S, Welcome to gun assembly/building 101, since all the checks that I have listed so far in this topic are basic checks/solutions that need to be done as the rifle is being built. 
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SOB bolt does retract too far,
swapped extractor, will add quarters although not seeing any brass marks to the rear of the ejection port. This is my third build, I am sorry most of these things seem like standard things to check and I was already aware of most of it, I appreciate your patience and helping me step by step, it helps quell my frustration and work through it logically. |
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Now worries, some build are almost plug and play, while others require a lot of fitting instead.
You just happened to get the latter with this build, So just kept at it plugging through what should have been checked/correct at first build, and will have the rifle right as rain sooner or later. Just think of it as a game of Finding Waldo (problems), that you are currently sucking at now. 
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I can only get 2 quarters in there before the bolt will not retract far enough to hold open on the bolt catch, even when the bolt is resting on the bolt catch the bolt face is about flush with the rear of the ejection port.
and that gives very little room/time for the bolt catch to pop up. |
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Quoted:
I can only get 2 quarters in there before the bolt will not retract far enough to hold open on the bolt catch, even when the bolt is resting on the bolt catch the bolt face is about flush with the rear of the ejection port. and that gives very little room/time for the bolt catch to pop up. Drop it down to a quarter and keep your eye on the back of the ejection port for signs of the brass tagging it on the way out. The bolt has to have over travel past the bolt catch in order for the bolt catch to be able to get up in time to catch the bolt on a empty mag. As for if you are not seeing any signs of the brass smacking off the inside edge /back of the ejection port, then start focusing hard on the extractor instead. As noted, if you have an over function problem, then the spent case rims should be bent to hell if the extractor is doing it job correctly. |
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Quoted:
1 quarter and a new extractor 20 round mag no malfunctions no more ammo, h414 gets here tue. Sounds like the rifle is now OK, and time to start onto the next build as you are wearing out the current rig.. As for the H414, you are aware that is a ball powder, meaning take a bit hotter primer to get a good starting burn. In 308 (match really), that is a little counter productive, since you want the mildest primer that you can use instead. If anything, the powder does meeter really well through a progressive press, but I don't subscribe to that, and instead load on a single stage with the powder dispensed out of of a electric scale dispenser instead and don't have the problem with extruded powder drops that you can run into with a volume dispenser. For me in 308, 168BTHP Serra bullets, 4064 powder, Fed 210M primers, and brass I open on, so long that they have been weight check/groups, and unified prep'd together. For 300 yards and less, around 2600FPS(+/_to find where the barrel is going to sing around that speed), while for 600 and beyond, push a tad hotter to buck wind instead (again, just looking for the next end of harmonic whip end point on the other side to set the speeds to get the barrel to sing again). . |
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Well the h414 was the only thing i could get an 8# jug of, it should work pretty well with the speer 180 gr sp, i have fed large rifle primers.
I also have some 150 gr sp blems from midway, tempted to try both h414. And h322 with those and see if either works better. Thats all the rifle powder i will have on hand. Once i use those up ill try something else out. I got no clue what i wanna build next, was actually considering a cheap old pieta cap and ball revolver |
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