I have a Spikes ST-22 upper that had a 16" barrel. It has about 1500 rounds through it. It was plenty accurate for the distances that I typically shoot - 60ft indoor range. I believe the barrel is a Green Mt. (did not pay extra).
I sent the barrel to ADCO to cut to 5.5" and thread. Now, accuracy went to crap. It patterns like a shotgun. Bullets tumble, and looking at a target shot at 30ft, there is marks in the paper showing the bullet went through sideways. I tried the ST-22 suppressed, un-suppressed, and with the suppressor mount removed (NOTHING screwed on the end of the barrel) - results are the same. The cut/crown/thread job by ADCO looks perfect to my eye.
Suppressed, it cycles perfect, no failures of any kind. Without a suppressor it is a single shot most of the time with most ammo - (I fully expected this).
I have tried the following ammo (every kind I found locally):
- FEDERAL Target grade performance (white & black box, says it is "ideal for semi-autos"), 40gn solid lead, MV 1200fps
- FEDERAL champion bulk pack, 36gn copper plated Great Wal of China Mart stuff
- Remington Sub-Sonic, 38gn Lead RNHP
- Remington/ ELEY Target rifle, no clue what weight bullet, but it's lead RN solid
- FEDERAL Spitfire (Gander Mt. exclusive), 31gn copper plated HP (my 16" ST-22 absolutely LOVED this stuff, never had a MALF and was laser accurate)
- Winchester 333 bulk pack, 36gn copper plated HP (this stuff sucks ass in anything I've tried)
I also brought my Bicentennial Ruger Single-Six, which also has a 5.5" barrel. Same time, same range, same distance and same ammo, accuracy was excellent with the Ruger.
I have one theory that I am going to check - barrel twist. Does Green Mt. make gain twist rifling for their AR barrels? Far fetched, I'll admit, but this thing really seems like it isn't stabilizing the bullets. I think the Single-Six should have a 1:14" twist, while the ST-22 has 1:16". I'm going to try to get to the outdoor range with my chrono, and I should see velocities pretty close between the two 5.5" barrels I would think...
Of course I am discontinuing use of the suppressor until I get it sorted out - don't need a baffle strike ruining my can
Any ideas from you guys. Thanks
ETA...
I think I may have discovered the problem. After I cleaned the barrel I took an absorbent tipped applicator (long wood handled q-tip). The q-tip part happens to be a little bit bigger than the bore, so it takes a little bit of pressure to get it into the barrel. While passing the tip through the bore, resistance can be felt and it spins with the rifling, until it gets to about 3/4" form the muzzle, where there is no resistance at all, then pull it the rest of the way out the bore and resistance once again at the muzzle.
I can see a darkened ring all the way around the bore when I shine a light from the chamber end. Can't really get a picture of it, but I drew the MS paint to show what I think is going on. Flaw in the barrel? It only has a little over 1500 rounds through it. The rest of the bore is mirror bright and shiny.