Six months ago I built an AR pistol utilizing a 7.5" upper in .556. I also built it with a chinese knock-off of the L.A.W. folding stock (brace) adapter. This requires a bolt extension plug fitted on the end of the bolt to prevent the bolt from coming out of the upper receiver when the stock (brace) is folded. The pistol ran fine on Lake City surplus .556 ammo but had function problems when using .223 ammo. Especially cheap, steel case ammo such as Tulammo. 7.5" AR pistols have always had a reputation of being finicky and ammo sensitive. It wasn't until I was cleaning my guns after a range trip that I really noticed that one of my AR-15 carbine was much easier to pull the charge handle back than all the others. Of all my home builds, the pistol had the heaviest pull on the charging handle. I removed the buffer spring out of the carbine and pistol and compared them next to each other and could not tell any difference such as uncompressed length or gauge of wire. I decided to switch buffer springs to see it that would make a difference in the pistol. Well, it did! I can now fire any .223/.556 ammo in my stash. Not a single FTF or FTE. Tulammo, Winchester, Wolf, and American Eagle all worked perfect. All this time I thought the pistol was over gassed or under gassed or bolt too heavy or not heavy enough. Never considered the buffer spring. Now I know. The one thing I did not do was to test the carbine with the heavier "pistol" buffer spring. I'm thinking that with a longer barrel, the carbine can overcome the added spring pressure due to the additional dwell time of bullet from gas port to end of barrel. Next range trip will tell.
