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S12 #2 is the piece of junk of the 4 I just bought.  It had just about all the problems of my #1 Century Arms conversion.  Couldn't cycle 3" on #1, had the left locking lug impacting the rear of the trunnion and jamming, hitting the upper finger of the hood and jamming, hard rubbing on the ejector, extractor spring too strong, ect.

 

Starting at the gas block and working back here's everything I did.

 

The plug was not ground down fully, only just enough to fit it in the block.  I sanded down the partially ground threads until most were not visible.  The Dremel sanding disks worked well and gave a pretty good polish too.  Also polished the flat surfaces. 

 

The 4 ports were not blocked, and was able to get a .062" probe down all but the rearmost port due to angle.  I did verify that the port was open with a piece of .032" wire.  I also noticed a burr in the barrel from the rear port, but was unable to remove it with a tornado brush.  In all ports checked with the .062 probe, the probe fit loose and would rattle.

 

On the barrel hood, I beveled the bottom of the upper finger and extension so that the bolt would give a glancing blow and not impact full on causing a stoppage.  I also deburred/rounded the edges of the hood to reduce the scratches on the shell rims.  The lower finger was also beveled to stop the shearing of the rim in that area.  The feed ramp mod was done at the factory on this one, so I didn't have to file it in.  I also noticed that the extractor was rubbing on the lower finger, and filing from the bottom of the ramp rearward let it climb the ramp smoother.

 

On the rear of the trunnion, I marked the highest point that the left locking lug could hit, and filed a larger radius so that the bevel on the front of the lug would hit the curve and push the bolt back down into the channel.

 

On the bolt, I checked hood locking action by pushing just the bolt into battery.  The initial locking action is very strong on this one compared to my #1 gun that has almost none.  I did notice some wear on the timing surface already from hand cycling the action and the initial test fire.  The cam had some wear on the finish, and showed a low spot that I stoned flat.  I also rounded the edges of the bolt face to provide glancing hits on the hood.  There's a couple of strong wear spots on the locking lugs that are a little rough, but I'm going to leave them alone for now, as any work there can change headspace.  The only work done on the lugs so far was removing burrs off the edges.  I also changed out the composite extractor spring for one that my testing found to work.  (Won't say who makes it or the specs, but Cobra's Custom has the info (let me know if I need to resend))  This spring is a little too strong at manufactured length, and needs trimming to .500" length.

 

On the carrier, I polished the bottom with a stone just deep enough to remove the finish.  I ran a 1/16" milling cutter down the ejector slot on the center line side of the slot removing about .032".  Big improvement in action smoothness.  The cam channel looks like it was cut by a non-machinist on his first day at the shop.  The walls of the cam channel were not smooth cuts.  This coupled with the low spot on the bolt gave a grinding feel as the bolt locked.  I stoned down the walls till the finish was removed from the low spots.  This gave the locking a smooth feel.  There was also a burr kicked up on the unlocking side possibly from overgassing caused by the proof test.  My #1 gun had burrs in the same area caused by overgassing. 

 

On the FCG, hammer got polished.

 

Result of test fire.

3" 1 1/4 oz waterfowl loads work on #1.

2 3/4" 1 1/8 oz waterfowl loads work on #1.

2 3/4" 1 oz slugs work on #1.

2 3/4" Mil Grd OO buck works on #1.

Federal 3 dram bulk works on #2.  Hip shoots with gun held loosely.

Federal 2 3/4 dram target loads cycle about 40%.

Other than the 2 3/4 dram loads, there were no malfunctions in 100 rnds of bulk pack.  The factory mag that came with this gun does not like to lock.  All other mags would lock with the bolt closed.  Shell rims have a deep knick comming from the right side trunnion area.  Not seeing the shearing anymore in the lower hood area.  The rest are minor scratches from mag feed lips, extractor, and feed foot/stripper.

 

On cleaning the bore, the burr on the rear port was removed by firing.

 

Now comes the fun of the pistol grip/922r conversion.

Edited by Capt Nemo
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I now have 5 total.  My #4 shoots perfectly at 2 3/4 dram out of the box.  #3 and #5 have a lot less bugs than #2.  I'll be going with the Pheonix tri rail and Kicklite stocks, Polychoke Deluxe, Tapco G2 and puck, and Magpul AFG for all.  As these are my bunker guns, I want to keep them uniform as much as possible.  Just not sure yet on guard and grip that I want to use.

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Agreed Evl. Best way I've found to solve said problem is seeing how many totally different directions I can go with the first five. The rest I'm savin' for what works out the best and for just the fact some are very rare.

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