The pages below are from AMCP 706-260.
Automatic firearms mechanisms are designed to unlock at pressures that won't rupture the cartridge. All gas operated mechanisms that I am aware of unlock when there still is residual pressure left in the barrel. This residual pressure is energy that is still available for work. Typically the action will unlock when the residual pressure is less than 650 psi.
In the first chart, the residual pressure drop curve is there for a reason. The pressure drop rate is a critical issue for a designer.
Timing is critical to the proper function of these firearms. Once the mechanism is desinged, a powder that is slow burning and thus has a high residual pressure, will cause function problems.
The second chart below shows that during the design phase, the timing of the mechanism is carefully calculated.
I have been involved in a number of discussions on my use of lubricated cases in semiauto firearms. To have an informed opinion, I needed to increase my understanding on the design of firearms. I have found that firearms design books are very rare. There are hundreds of “eye candy” books, with pretty pictures of firearms, but very few books that provide the information to understand operation and design principles. I have found a few and hope this list is useful for those looking to expand their understanding of the fundamentals of firearms design.
1. “Technical Notes, Small Arms Design”, Author: John G. Rocha , available from Armalite (800) 336-0184, stock number NA1085 $12.50 . This soft cover pamphlet is a copy of the course material handout to a firearms class taught at Rock Island in the 60’s. Very interesting, a good number of formulas, but still very readable.
2. Brassey’s Essential Guide to Military Small Arms, Design Principles and Operating Method, Author Allsp and Popelinsky, Brassey’s Inc, 1997. IDSA Books , (937) 773-4203 price was lower than Amazon.com’s. This starts off simple and ends up very technical. This is a serious book and the final sections really require education in math, science, or engineering.
The above two books are the only ones that I have found that are currently in print.
3. The Machine Gun, Volume IV, parts X & XI, LTC George Chinn, pub 1955. Out of print. This book should be the absolute first book to buy for someone who is interested in the principles of automatic weapon design. Really an excellent statement of principles. It is a comment on the general state of technical ignorance in our society that this book is extremely rare, but the general public is not that much interested in it. A Gun Show book dealer told me more people wanted the volumes with the pretty pictures of old guns!
4. The Bolt Action by Stuart Ottenson. First edition by Winchester Press 1976. Ottesnon later added a Volume II which came out in a two volume edition by Wolfe Publishing in 1985. These books are very non mathematical for a general audience, but the principles, particularly expounded in Vol 1 on the Mauser 98 are, in my opinion, fundamental to the understanding of bolt action design.
5. AMCP 706-260. Engineering Design Handbook: Automatic Weapons. Out of print. The most technical and mathematical design book I have found to date. Assumes a high level of knowledge in firearms design, mathematics, and Mechanical Engineering. A technical degree, preferably in Mechanical Engineering is really needed to attempt to understand the presented material. This was created in the early 60’s, and reflects the designs, and the design knowledge of the day.
6. AMCP 706-252 Engineering Design Handbook: Gun Tubes. Out of print. Very interesting, not limited to small arms.
AMCP stands for Army Material Command Pamphlet. There were about one hundred AMCP Pamphlets covering information ranging from Statistics to Automotive Design. A copy of AMCP 706-260 and other out of print AMCP pamphlets can be ordered from NTIS at 703-605-6000. These are not cheap, they want $45.00 to $150.00 for some of them.
7. Small Arms & Cannon, Smith and Haslam, RMC of Science, Shrivenham UK, 1st Edition, 1982, Brassey's Publishers LTD. No idea for a source really excellent read.
Firearms design books are so rare because the market is so limited. I would suggest to anyone who has a technical background, and is interested in firearms design, to acquire those books in print. Because once they are out of print, you may never find one. Or you will have to pay, like I did, about $100.00 for Chinn’s Volume IV. And be happy to get it, because the price is now $250.00! SlamFire.