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Posted: 12/28/2016 11:43:00 PM EDT
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So I recently picked up an 11.5 upper in the EE, it started as a 16" LMT M4gery that was sent to ADCO and cut down to 11.5, possible gas port mods done. It is running a cut down FSB.
During a rushed sight in trying to beat sunset, I had 2 separate bolt over base FTF on the last rounds of 2 mags, both times the bullet tip was jammed into the top of the chamber, setting the bullet back all the way flush with the case mouth, and the bolt gouged into the side of the case and came to a stop. It's throwing brass mostly forward, around 2 o'clock as far as I can tell. I'm using a lower that I've had for years, my primary go to lower, running a standard carbine buffer and spring. Never had issues with 16" carbine and ML gas uppers. I've also used this PMAG for a couple years without issues.  I'm assuming it's overgassed, the recoil seemed to be on the harsh side (relative to what you'd expect). I'm trying to avoid an adjustable gas block on this one for simplicity and durability. This is the first time I've ran into "tuning issues" so thought I'd go for a second opinion before I start tinkering with stuff! |
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For process of elimination try some other magazines, as you mention that it did it on the last 2rds in the magazine twice. Next look at the feed ramps, as if it's having a feed issue on the same side consistently then the feed ramps could be misaligned or have deformities / flaws. As already mentioned you can try slowing the cyclic rate down by adding a heavier buffer and/or heavier action spring (buffer spring). If you suspect it being over gassed, can you measure the gas port, as 14.5"-16" M4 barrels are generally .063"-086" for the gas port, where the 11.5" barrels are generally .081"-.094" for the gas port.
CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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What mags in play???
The 11.5" sbr is going to cycle harder/ have more of a jolting effect when the buffer tags the back of the receiver extension during cycle than say a 16"/20" barrel rig, and some shite mags will not control the top round in the stack correctly to get it tight up against the bottom of the feed lips of the mags, for the correct attack angle at round strip out of the mag, and this nose up round above the feed ramps is a common tail tail sign of such problems. If a known good type mag still in serviceable condition (or new), them pull the mag apart to give it and all it parts a good CLP. Hence new mags often have shop debris in them that will cause the follower to bind to the side of the body to cause the problem, while older mags that have used without cleaning, fouling inside the body and parts that will cause the same problem isntead. As for a quick test, single load a round into the mag, insert the mag and charge the round, and while leaving the empty mag still in the well, fire the round. If the bolt locks back, and the spent case is ejected out of the ejection port 10' to 15', the the cycle is fine. If the spent case is just dribbling out with the bolt still locking back on the catch, could be a extractor problem, of just over function with the bolt way unlocking too soon, and the spent case still too residual bore bound to the chamber for clean pull on extraction instead. If the later is the problem, the the tell tail sign will be the spent case rim bent all to hell. If the problem is just the extractor (it's tension and grasp to the case rim), then the rim of the spent case will not be bent to hell instead. Note, if the chamber is not mirror smooth, but instead has reamer chatter marks, or you are not scrubbing the chamber correctly during cleaning, then these will give a false over function bent rim sign, with the problem being chamber conditions, and not the bolt unlocking too soon instead. As stated, the SBR cycle is going to be more violent cycle than a longer barrel rifle, and some time it just boils down to a questionable mag not being able to keep up with the more violent cycle of the SBR instead (recover of the top round before the bolt goes to strip it). Yes, you can go to a heaver buffer to slow the cycle speed down if the rig is way over functioning (bolt unlocking way too soon), but more often when the rifle is cycling correctly to begin with (bad mags) it just ends up with the rifle short stroking instead. |
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Sorry for not getting back to this thread, I got busy and forgot about it! I haven't had a chance to mess with it anymore. It seems to be just overgassed, runs OK but harshly, occasionally the very last round will do the FTF. I've had the problem with USGI & PMAGS that function fine in everything else.
I wish I had measured the gas port while the cut down FSB was off so I could switch rails! |
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