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Posted: 10/4/2015 10:06:36 PM EDT
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I need help to figure out what to change/fix to get this upper working. It is a 16.5" barrel with 1-18" twist and carbine gas system. that Marty ter Weeme made for me. I've had it a few years but just now had a chance to assemble it into an upper. I loaded 325 gr FTX over Lil Gun. I started at 27.5 gr and worked up to 29.5 grains in 0.5 gr increments. 27.5 and 28.0 gr showed very faint swipe marks. They would eject the case, but not lock the bolt back. The heavier loads showed progressively heavier swipe marks. They ejected the case and cycled the bolt far enough to catch the front of the carrier on the bolt stop, but not far enough to lock the bolt back properly.
The failure to cycle makes me think that I should open up the gas port. But the heavy swipe marks makes me think that I need to find a way to reduce the gas and slow down the bolt unlocking. What's going on? What steps should I take to find a solution? |
| Sounds like a timing issue. You might want to give Tony at Tromix a call. He has been very helpful with problems in the past. Might be too small a gas port. But there are so many other things. Did you put in a heavy spring or buffer?? Any other changes from a standard ar15? |
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I'm running a std. carbine buffer and a F/A carrier. I'm not sure about the spring. The mag is a Lancer modded by Wilson Combat for 458 SOCOM.
Primers were Win Large Pistol. The heavier load increments showed some primer flattening, but nothing I'd consider dangerous. Outside edges of primer cups still had some rounding left and showed no flow. Marty sent it to me with the gas block installed because I was planning to use it with an MGI upper. So I did remove the gas block to install the barrel in the receiver but I did not measure the gas port. I plan to remove the rail handguard and gas block to measure the port. What size should the port be for a 16.5" bbl with carbine gas system? Marty also supplied the bolt head which he had opened up from 5.56 to 458. I probably should have disassembled it and smoothed any sharp edges on the bolt face, extractor and ejector before shooting it. That may account for the swipe marks on the case head. There was no damage to the rims and no marking on the case body. Just swipe marks/gouges on the headstamp part of the case head. |
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It may have come from Tony. Marty worked with Tony at Tromix on the original design. Also the winlpp have been problematic at leaving unburned powder in the chamber. Most who load for the 458 like using cci350 large pistol Magnum primers. Seriuosly give Tony a call or get registered in the 458socom forums. Tony hangs out there much more than he does here. He may chime in to.
So has this happened more than once? Clean the chamber and run one at a time and check for powder residue after the shot. That is the first thing to try. It only takes one particle to cause things to jam up with tight fitting chambers. |
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Brazos,
Not really a suggestion, but perhaps clarification. Marty built my 20" barrel with carbine gas, for my MGI QCB, many years ago. It has run like the proverbial top. And at 20 inches, it probably has more port pressure than your 16.5, assuming everything else the same. perhaps removing the gas block to have a measurement of the port size or checking alignment might be useful, but I would suggest not doing any reaming of the port, until talking with Tony. I have not checked to see if his website (Tromix) has a direct link for contacting him, but he is fully in the 458S game, was the original builder and tester of the 458S and should be able to get you squared away. FWIW, I added a light weight 16" barrel from Tony earlier this year for use in the same MGI upper, and it too runs perfectly. Craig |
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Tony has been back fully in the .458 SOCOM game for over a year, turning out everything from barrels and bolts to complete uppers in barrel lengths from 8 to 16 inches at prices that whip the snot out of everyone else. He is also doing .375 SOCOM and making forming and reloading dies for the .375.
And yes, there is a direct contact link on his web site. |
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Quoted:
Brazos, Not really a suggestion, but perhaps clarification. Marty built my 20" barrel with carbine gas, for my MGI QCB, many years ago. It has run like the proverbial top. And at 20 inches, it probably has more port pressure than your 16.5, assuming everything else the same. perhaps removing the gas block to have a measurement of the port size or checking alignment might be useful, but I would suggest not doing any reaming of the port, until talking with Tony. I have not checked to see if his website (Tromix) has a direct link for contacting him, but he is fully in the 458S game, was the original builder and tester of the 458S and should be able to get you squared away. FWIW, I added a light weight 16" barrel from Tony earlier this year for use in the same MGI upper, and it too runs perfectly. Craig The soot mark under the gas block indicates good alignment between the barrel port and gas block. The port is ~0.100", which sounds a bit small to me. The number I've seen for 16.25" barrels with carbine gas is 0.110" (#35 drill bit) which would give 17% more flow area. I've used the contact form on Tony's web site to request what size port he is currently putting in his 16.25" barrels with carbine gas. |
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Brazos,
There you go, straight from the source. But, as you know, opening the gas port is certainly a one way street, for the most part. I have a 300blk barrel for my MGI that I did have to enlarge the port, so it is not hard. The but part, as you are seeing conflicting signs as to the direction you need to go, if I were in your shoes the first thing I would decide was how important the 458S was to my battery. If decided that I wanted to keep it around, I would probably order a bolt from Tony (looks to be about 110 dollars). This might help confirm the brass marks as something other than high pressure. And if still seeing indications of low port pressure, would give you a spare bolt. The next least costly thing I would do before opening the port would be to try and find a box of factory rounds. Your loading information does not look suspect, but shaking the options tree completely would be something I would want to do. I am assuming you have the larger gas block shoulder like my TJ build. If you do end up over gassed, getting an adjustable gas block might be a chore. Chewed up brass with the 458S is certainly not desirable for me, as my pockets are not that deep. Craig |
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Got it running without opening up the gas port. I'd been using ordinary gun oil in the carrier and it wasn't doing the job. After firing, the bolt carrier and bolt rings were dry. When I ordered the SBR factory ammo, I also ordered some Slip 2000 EWL. This may not be the best carrier lube, but with a listed temperature limit of 1250F, I figured it would stay put on the rings.My 29.0 gr Lil' Gun reloads also functioned fine with a COL of 2.200", but showed more pressure signs that the SBR ammo.
So I have loaded a few more rounds of the 29.0 grain load with the COL extended to 2.230". I also loaded a ladder with RL7 from 34.5 gr to 37.5 gr in 0.5 gr increments and with a COL of 2.230". I will fire all of these the next time I get to the range to see if I have a mild pressure and reliable cycling load worked out. Is Slip 2000 EWL considered about the best lube for inside the carrier, or are others more recommended? |
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Brazos,
Neither my TJ nor Tromix 458's are particular about lube type. I do have an old bottle of SLP that I sometimes use, but more likely I will just be using CLP when out shooting. If the lube has indeed "fixed" the problem, I would hazard a guess and say that you just need a bit more shooting on the bolt and carrier with the SLP to help get things loosened up a bit. Once this happens, I would further guess that the upper may not care about which lube you settle on. Craig |
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Quoted:
Brazos, Neither my TJ nor Tromix 458's are particular about lube type. I do have an old bottle of SLP that I sometimes use, but more likely I will just be using CLP when out shooting. If the lube has indeed "fixed" the problem, I would hazard a guess and say that you just need a bit more shooting on the bolt and carrier with the SLP to help get things loosened up a bit. Once this happens, I would further guess that the upper may not care about which lube you settle on. Craig I was only talking about using the Slip2000 EWL on the back of the bolt where the piston rings are and the inside of the carrier where the piston rings ride. That's the only place where temps too high for regular CLP are likely to occur. Everything else I just wipe down with a 12 gauge patch wet with regular CLP. As you say, maybe once the piston rings wear and seat completely to the carrier bore it may not be as lube sensitive. I remember rebuilding engines as a kid and needing to take about 500 miles for the rings to seat after rebuilding an engine. |
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