Flabbergasted

Here's a pic of the gun, just in case you wanted to see it.

I finally got around to addressing the issue that I was having with my gun. I wanted to try one possible fix at a time to pin point exactly what the problem was so I purchased a Tubbs buffer spring from LaRue Tactical, since it was recommended to buy there since they have the best price. Once I installed it I noticed a significant reduction in the spring tension when I cycling the bolt. The previous buffer spring was part of an Aero Precison M5 lower build kit and to me the buffer spring felt really stiff, but since this is my first 308 build I had nothing to compare the buffer spring to. I did however read a review someone wrote who was having the same issue as I was when they used Aero's buffer spring. They said once they switched to the Tubbs the problem went away. I took the gun out to the desert yesterday with a buddy of mine and shot 40 rounds of some cheap Monarch 150 gr ammo. I used the same magazines as I did when I had FTF and failure to lock the bolt back on the last shot. All 40 rounds cycled perfectly! No FTF and the bolt locked on the empty mag every time. I'm guessing the buffer Aero Precision spring was stiffer and may have been better suited for a gun with more gas. I'm just glad it was as simple as swapping buffer springs.

When I installed the mag catch I used a pencil to push the mag release button in past the receiver and proceeded to tighten it so the post sits almost flush with the button, like on an AR 15. I soon realized that that would not work on an AR 308. So I in screwed the mag catch until it was tight enough to hold the mag but not too tight to where I would have to push the button in past the receiver to get the mag to drop out. Just enough so I could drop mag by pressing the button as far in as I could with my finger. The release button looks like yours in the first pic. I haven't measured it yet but it looks the same as yours. I even loosened and tightened it a couple of times to make sure it was tightened the right amount. When the magazine is locked in I can move it back and forth with my hand, but just moving the gun around without touching the mag it does not move. Is this how tight the mag should be? I have nothing to compare this to since this is my first At 308 build.

Thanks!

The bolt catch seems to move freely. When I insert an empty mag and pull the charging handle back the bolt catch holds the bolt. I know thats different from when the gun is being fired but it seems to move freely. I was reading a review for the Tubbs spring and the receiver said that they were having the same problem as me and they were using the same lower build kit. They said the Tubbs spring solved the issue.

The mag catch is is just too small. It was part of an Aero Precision M5 lower build kit. I had to screw it on so little in order for it to be able to use the mag release button without using a pencil every time to push the button in past the edge of the receiver. The threaded stem on the mag release is just too short. Aero even sells an extended button to remedy this problem I think they just need to make a different mag catch for the AR 308. I think I'm just going to ditch the mag catch and get the armalite one that was suggested. The threaded stem is longer.

Wash - Yes my mags wobble back and forth when they are locked in. I think that's what I'll do is methodically work my way through each part of the gun to check and see exactly where the problem is starting with the buffer spring. Once the bolt is locked back I'm able to pull it back about 1/4" with the charging handle. See the pic.

Yes, the buffer is 2.5". Its the standard 308 car buffer. If I'm having trouble with the gun locking back after the last round wouldn't adding an extra strength buffer spring to the equation make if even harder for the BCG to lock back on the last round? Would just a standard buffer spring be better in this situation?

I think my gas tube is the proper length, at least it looks like it to me. I just looked at my magazines and they are Gen 3s. I'll be buying some different types of magazines next week and I'll try them out. Having more magazines is never a bad thing. The Armalite mag catch kit is out of stock on Armalite's site right now but I plan on buying it when it becomes available. It seems like the Armalite mag catch will be better than the one I have on there now. The mag release button is threaded on by a few threads. It seems like it is too short for the width of my gun. I'll start with those areas to see if that solves the issue. I'll most likely just replace the buffer spring while I'm at it since its cheap. I'll let you all know what the results are. Thanks!

I'll have to get some other brands of mags. I'm not sure if the pmags are gen 2 or gen 3s. I'll look into that mag catch you mentioned. If it's not the recoil system or the mags or mag catch, what else could it be? The only other thing I can think of is the gas port size.

All factory ammo. Federal GM Match, fusion, perfecta, and others. It happens with all types. Mags are 20 round Magpul mags.

Inside depth is 7". The recoil spring is definitely different from an AR 15 spring.

Joe Carlos who writes articles for the American Gunsmith Association has been using Loctite and shim stock when installing barrels for quite some time. He's conducted his own tests and has noticed accuracy increases when he uses it. I've used loctite 620 on one of my builds due to the barrel fitting loose in the upper. It shoots well. Not sure if it is due to the loctite or not. I have removed a barrel with after the loctite has cured. It sucked. Had to use a heat gun and beat the barrel out of the upper, but it came out.

Yes.

Thanks for the info.