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Posted: 1/8/2014 7:29:57 PM EDT
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I just put together a new rifle and have been test firing it using my hand loads. The loads are known-good in other guns, and are reasonably hot being at maximum load per published .223Rem data. The rifle cycles enough to eject the spent case (at about 4 o'clock) and feed the next round, but not enough to lock the bolt back on an empty mag. Testing with M855 ammo causes the bolt to lock back no problem (presumably 5.56 = more pressure), so I have ruled out the magazines and bolt catch - I'm pretty sure this is a marginal undergassing situation.
The build is a little bit of a thorobred, so I am not totally surprised to be working through some teething problems. Here is a list of the important components: * BCM 18" SS410™ Barrel with Rifle Length Gas (stripped) 1/8 Twist (5.56 NATO Match USMC SAM-R Chamber) * Syrac adjustable clamp-on gas block * JP Low Mass bolt carrier group * Collapsible stock with standard CAR buffer and spring * GI FCG * Billet upper and lower. Upper is a new side-charger from Gibbz Arms The BCG is well lubed, and I even tried switching it our for a regular M16 BCG. I pulled the gas block off and double-checked alignment with the gas port. The gas port mikes out at right around 0.100", which is at the bottom end of the MK12 SPR spec range (0.100-0.105" IIRC). I hear JP recommends 0.104". Could my gas port really be too small, even with the lightweight BCG? Or might the problem be with the Syrac block... turning the gas screw, from flush with the block, all the way out to the 4-thread recommended maximum gas does not appear to have any affect on ejection pattern, so Im wondering if it is constricting the gas flow somehow? Can anyone suggest anything else I have not considered. I regard drilling the gas port out as a last resort, as I may decide to go to a non-adjustable block later, and I don't want to be overgassed. If anyone can share their experience of the Syrac adjustable gas block, that would be helpful too. Here is a photo of the patient: 
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Load specs:
POWDER: AA2230 CHARGE: 25.0gr (published maximum) PRIMER: CCI #400 SRP BULLET: Hornady 55gr FMJ or Nosler 55gr Ballistic Tip (both fail to lock open) No visible signs of distress on the primer or case. Like I said, this load (indeed this batch) has proved to be reliable in many other rifles, so I am skeptical the ammo is bad, but I acknowledge it is always a possibility with reloads. One update: Reading more about the Syrac gas block, it seems that it is not designed to sit tight against the barrel shoulder as I had assembled it, but instead to have a gap equivalent to the handguard retaining cap. I measured a cap I have on hand at 0.030", so have adjusted the positioning of the gas block. I'll test it again this morning and report back. Hopefully it is this simple. |
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Too short of a bolt catch on the magazine side (separate the upper from lower and insert an empty magazine and see how well the follower pushes the bolt catch. mate the upper and lower, put in an empty magazine, and pull the bcg back all the way to see if it locks on its own like it should with an empty mag)
Gas port too small (Doubt it being BCM but check the size anyway) Misaligned gas block Gas block leaking around the barrel from not being pushed back far enough or not tightened enough (put red locktite on the barrel under the gas block so that it seals around the gas port and torque the set screws down using red locktite also) Gas tube not patent (run a pipe cleaner through it) Gas key leaking (put a straw in the gas key and blow, looking for bubbles from the oil you have put around the base. torque the screws and seal it with red locktite) Resistance on the gas key and gas tube articulating (remove the bolt, put the carrier in the upper while the upper is flat, tilt upper 45 degrees and the carrier should slide to the end of the receiver freely. if it drags then you have a bent gas tube or key) Bad rings (put the bolt back in the carrier but leave out the cam pin. you should be able to hold bcg by the bolt and the carrier not fall off. if it does, replace your rings. there is no solid evidence that ring alignment causes a problem) Too heavy of a buffer Too long of a spring (I have encountered "carbine" length springs that were 2 coils different in length. Check your length spring against another and shoot it with the shorter one.) Oil everything appropriately Report back when you've checked these and ran some test fires. It sounds as if you're right on the edge of short stroking and if you can push the bcg back a bit more you'll have it locking when empty. |
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Quoted:
Load specs: POWDER: AA2230 CHARGE: 25.0gr (published maximum) PRIMER: CCI #400 SRP BULLET: Hornady 55gr FMJ or Nosler 55gr Ballistic Tip (both fail to lock open) No visible signs of distress on the primer or case. Like I said, this load (indeed this batch) has proved to be reliable in many other rifles, so I am skeptical the ammo is bad, but I acknowledge it is always a possibility with reloads. One update: Reading more about the Syrac gas block, it seems that it is not designed to sit tight against the barrel shoulder as I had assembled it, but instead to have a gap equivalent to the handguard retaining cap. I measured a cap I have on hand at 0.030", so have adjusted the positioning of the gas block. I'll test it again this morning and report back. Hopefully it is this simple. I believe I read somewhere that a standard hand guard cap is 0.025 thickness just in case your measurement helps but does not fix the problem. Sounds like you are on the right track. |
PROBLEM SOLVED: Indeed, the gas block was too far back and not getting all the gas. Moving it away from the barrel shoulder about 0.03" now causes the bolt to lock back on an empty mag every time with my handloads. I wish Syrac would put this little nugget of wisdom on their instruction sheet. With the gas block opened up all the way, the bolt/buffer stack still feels like it just barely hits the back of the receiver extension, but I hope things loosen up as I put more rounds through the gun (only about 25 so far). I'll know for sure if the gun is 100% when I run it in a 3-Gun match on Saturday... those of us who compete know guns show their true reliability ONLY after the timer beeps. 
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Quoted:
PROBLEM SOLVED: Indeed, the gas block was too far back and not getting all the gas. Moving it away from the barrel shoulder about 0.03" now causes the bolt to lock back on an empty mag every time with my handloads. I wish Syrac would put this little nugget of wisdom on their instruction sheet. With the gas block opened up all the way, the bolt/buffer stack still feels like it just barely hits the back of the receiver extension, but I hope things loosen up as I put more rounds through the gun (only about 25 so far). I'll know for sure if the gun is 100% when I run it in a 3-Gun match on Saturday... those of us who compete know guns show their true reliability ONLY after the timer beeps. Between the rifle self matting/self polishing in, and the chamber getting a little more polished/looser through live fire and cleanings, the rifle will be right as rain in no time at all. As for the load, trust me, you are no where near max pressures (but do not bump the power charge until you can get some loads pressure tested). I have some M-193 loads bases on 2230-C, and run a lot hotter that you are running to hit Nato pressures with the powder instead (yes, getting 3250fps out of a 20" barrel with the 2230-C reloads). As for the Boat tail design bullets, might want to check the jump to lands for loads coming out of the mag. Truth is, if the BT design bullet has to jump more than .010" before embedding into the rifling at ignition, then the barrel be more accurate with a flat base bullet instead (even for bucking wind out to 300m). |
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Quoted:
And what bullet would not exceed .010" jump and still feed in a .223 AR15 magazine? I load 52 thru 75 gr and all exceed .010" mag fed. Thanks!! A match short throated barrel from someone like Compass lake that you either send them a round so they can set the throat for it, or tell them to throat the barrel for a .003 jump to lands with say Mk-262. But here is the tricky part, if you throat for something like Mk-262 with its longer ogive, then 55/62 grain ball ammo with a shorter Ogive and loaded to the same OAL, is going to embed into the rifling at loading. Also to add, when ordering a barrel this way, your start load will have a shorter OAL that what the data would tell you to load the round OAL. This allows to lengthen the OAL of your reload to compensate for the normal plasma erosion of the throat of the normal life of a barrel so you can maintain the an ideal short jump with your specific ammo; will still being able to come out of the mag. As for how much shorter to compensate for the throat erosion over the life of the barrel, depends on how the rifling is produced. With a broach cut rifled barrel, barrel life expectancy is around 4K before the barrel just doesn't want to group tightly anymore and the barrel needs to be replace. With a button rifled barrel, the barrel will start to drop off the mark around the same count, but does so gradually instead, and may be good for around 8K rounds before it time to replace the barrel since it will not hold a tight group instead. |
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