When a cartridge is fired, high pressure gasses push the bullet in one direction down the barrel, and the (now empty) cartridge case is pushed in the opposite direction out of the chamber. In a simple blowback firearm operating system, there are no locking bolt lugs, or any other mechanical locking or delay mechanisms, to keep the breech closed during firing. The most common problems that come up with 9mm blowbacks are ejection issues and chambering/feeding issues (see below for more info.) Keeping everything clean, lubricated, and carefully inspected should help prevent a number of potential problems. Simple blowback systems get dirty quickly and may be rough on parts. Aluminum cased ammo is generally not recommended in simple blowback systems. 9mm buffers generally weigh from 5.5 oz.
Bolts available on the consumer market generally seem to weigh 14 oz. is a bolt + buffer weight range that seems to work well with the vast majority of configurations and ammunition types. Extended length buffers (about 4.0” long) are available that are designed to eliminate the need for a buffer spacer. It also helps prevent fired cases from getting jammed in the fire control group pocket of the lower. A buffer spacer limits the rearward travel of the shorter bolt and helps prevent bolt catch damage. The 9mm bolt is about 0.75" shorter than the 5.56mm bolt and carrier.
Solid steel buffers were used at first, but they switched to sliding weight buffers after reports of out-of-battery discharges from bolt bounce. The Colt-style magazine is based on the Uzi mag, with the mag catch moved and last round bolt hold open (LRBHO) capability added. sliding-weight buffer (bolt+buffer recriprocating weight of 21.5 oz.), an aluminum buffer spacer (0.75") installed in the back end of the recoil spring, a standard carbine spring, and carbine buffer tube. The original AR-pattern 9mm simple blowback operated SMG/carbines were developed by Colt. I know I would have appreciated having all of this organized and in one place, so here it is. This is information I found while researching all the problems. The first 9mm AR that I assembled had many issues I had to work through, requiring many hours of research and experimentation. The accuracy of this information is not guaranteed or warranted. Do not perform any action based on any of the following information. This information is intended solely for entertainment purposes only. There you can browse or easily jump to individual sections for convenience. All of this information, with current updates, is available on my site under "Everything about the AR9". I will no longer be updating this thread as new information becomes available.
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