First and foremost a good read of the original post would have been helpful.
Some good ideas, some bad, some contradict.Dibs50 wrote:While I've owned AR's for years, this is my first pistol build. During function testing, I had FTE/FTF issues. I've searched this site and suspect that it's an over - gassing problem. I've attached some photos and hope someone could assist me.
My build took some time as I gathered the parts as I could afford them. Here are some details :
10.5" BA performance barrel
Tactical Machining upper
ALG 10" EMR V-3 handguard
Carbine length gas tube
Non-adjustable gas block
KAK Industry Flash Can
Anderson BCG
AERO Precision lower
Carbine buffer (2.9oz) and spring
I'm shooting handloads
147gr M80 pull downs over 17gr of Lil Gun. COAL: 2.145
Problem: I'm getting both a FTE and a FTF. Cycling one at a time, the BCG locks back on an empty mag. It has worked with 2 & 3 rounds in a mag. More than that and I start having problems. I've tried both 20 & 30 rounders, Military and civilian (Magpul) magazines. I don't see any signs of pressure. Should I be looking to get an adjustable gas block? Any help would be greatly appreciated.
The pistol described has carbine gas and with the carbine buffer is configured correctly. That does not mean it can't be over gassed, as many would believe, even without a suppressor. The second part of too much weight for the magazine to feed is probably correct and slowing down the bolt speed would be the answer.kilibreaux wrote:The gun isn't clearing the fired case before picking up the new round. That means weak ejection, but not likely "undergassed" since a pistol length system puts plenty of gas into the system, unless the port is too small. A bolt that locks back on a single round fired, or works with three rounds but starts to act up on more means stack friction is overcoming bolt throw. The primers appear well flattened indicating adequate pressure, as evidenced also by the striations showing on the fired case jammed in the action, but clearly, the fact that two cases are present - one empty and one full, means the action failed to clear one before attempting to chamber the other.
You might need to experiment with a faster powder that moves your peak pressure curve closer to the port, or you can do as so many of us have and mount a Krinkov "brake" which is really a muzzle "booster" that increases back pressure and makes short barrel systems work. If you do go the Krink route, first, don't buy some other, high-dollar brand, stay with the "cheap" Krink, then shorten the straight tube section behind the cone-shaped funnel to gain clearance between bore exit and gas chamber. The Krinkov style "brake" was designed to make short-barrel AK's function and it will do the same with an 8 - 10" AR "pistol."
With the Krink "brake" you can then give some consideration to a heavier buffer so slow your cycling mass.
Changing to a faster powder could help, but not for the reasons stated. A faster powder than Lil'gun will have a higher peak, a faster rise and fall. This will actually move the peak away from the port and have less pressure at the port in a carbine gassed system. In this rare case it might solve the cycling problem if the powder was fast enough, or if too fast it will not cycle at all. The other thing it will likely do is lower the velocity, since most powders faster then Lil'gun will max out with pressure before achieving the same velocity.
Adding anything to increase pressure at the muzzle, therefore adding back pressure for cycling will increase the problem at hand.
The contradiction is suggesting a powder change to decrease port pressure, then adding a muzzle device to increase back pressure makes no sense that I can think of for this application.
Again faster powders and heavier bullets will only enhance this problem and or create others. Going to a slower powder, especially if going to a heavier bullet, would give less pressure and higher velocity in this configuration.kilibreaux wrote:Remember that 300 Blackout produces high pressure with low volume so your port pressure becomes more problematic. Examine a powder burn rate comparison chart and consider trying powders faster than you are using, or go with heavier bullets, or go with lighter bullets in the 110-125 grain range and go for velocity which means powder choices and charges that will cycle the pistol system without a hitch.
