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I have a CAA 6 position collapsible stock with a stock buffer and spring, my issue is the bolt will not hold open, on the last round? Assuming my spring was to strong, I clipped 2 coils, no difference. I have used 3 types of ammo Hornady, Hornady Black and Remington no difference?????

I'm no expert... I have an APF 16" barreled upper on a Core15 lower with Hogue adjustable stock. APF told me the stock carbine spring and weight would work fine, it did. But I installed a Armaspec H2 captured spring for it and it is very nice , brass comes out at 4 o'clock and no problems with locking on empty mag. I have shot Hornady, Buffalo, Black Butterfly and another brand of ammo all have locked on empty mag. I also have 4 or 5 mags made by 2 different manufacturers. . Have you tried a different mag? Can you get it to lock by cycling by hand? Check gas block and make sure it is in correct position and gas port is not blocked a little. Catch spring weak?

Again I'm not an expert on AR15 rifles so excuse my lack of proper terminology or understanding on how things work,,, check entire gas system for things like loose parts, rigs wore out or all rings aligned letting gas bypass.

To start, whenever you test fire a new build, always load just 1 round in the mag, then charge and fire. Do this a few times to check for bolt hold open, proper cycling, ejection pattern, and check that trigger works properly (don't need a full auto situation).

What is your build? more info the better.

With your rifle so far, did you notice the ejection pattern?

Does the bolt go back all the way, or is something impeding it?

Do you have heavier buffer's to test?

Did you check the gas block for proper alignment?

Is there something obstructing the gas port?

Is the gas tube properly installed?

Is the bolt catch actually catching the follower of the mag?

I would install a new standard carbine spring.

Glad Bmt85 has responded. Having a little time today I did a search and read up on locking issues. One interesting problem I saw was one guy had a Rifle length spring instead of a carbine length spring in his adjustable stock..once he installed the shorter carbine spring he was good. Others had a Pmag issue, snug/tight bolt catch issues, etc... so it's not always a gas related problem. I have not assembled an upper or lower myself, but may put one together in the spring if one of my buddy's has time to help me out, he has built a few.

Since it is the follower that locks the bolt open after the last shot, I would check this first. I had this problem on one mag, and when I compared the follower to one in a mag that did lock back, there was a physical difference in the configuration at the rear. I changed out the follower for one with the same configuration that worked, but I have not yet had a chance to test it.