View Full Version : Cyclic Rate and Magazine Timing
April 9, 2002, 03:50 PM
I'd like to discuss the effect of cyclic rate and feeding devices. I recently read about the Russian Abakan rifle that shoots two rounds at 1600rpm cyclic and then fires the rest of the rounds at a more subdued cyclic. In the description of the system, it stated that a special loading system had to be divised whereby the second round was pre-positioned as the magazine would not feed rounds at that rate.
Now, I know that the Mac-10 would feed rounds at about 1200rpm cyclic and I believe the open bolt Thompson would approach that often. The port firing version of the M-16 would also approach this 1200rpm cyclic. Is there an upper limit to traditional box magazines in terms of cyclic rate?
I had the idea floating around in my head to increase the cyclic rate on the M-16A2 so that the 3 round burst would happen quicker and therefore produce smaller groups. Just an idea.
April 9, 2002, 05:33 PM
In most cases, the bolt moves too fast for the magazine spring to move the next round into place.
To find the fastest rate at which the weapon would function, you'd have to compare how fast the bolt moves and how fast the mag spring can push up, of course baring any thing that would slow it down...
April 9, 2002, 07:16 PM
Agreed. Might I assume that that also means the fastest a magazine can move the rounds RELIABLY under a variety of conditions with a variety of ammo.
April 9, 2002, 08:13 PM
I think you got the point on magazines and timing.
There's a great video at Springfield Armory National Historic site which shows a firearm firing in the full auto mode. You can watch the firing, extracting, ejecting, feeding. It does this several times until jam! An action can only feed as fast as the feeding device. Timing the two is tricky and this is one problem the U.S. Army faced when it tried to convert the full auto versions of the M-1 to take the BAR magazine. The light bolt of the M-1 was simply too fast and the BAR magazine spring could not keep up with it.
Garand proposed two modifications. Either lengthen the M-1 receiver 1/4" to allow more distance for the bolt to travel (thereby giving the magazine more time to push up a fresh cartridge) or modify the BAR magazine to be faster (in which case it would not longer be compatible with the BAR - thereby defeating the original concept of making the two weapon systems compatible). Remington finally came up with the best solution when their engineers put a buffer into the recoil guide spring. Said buffer would slow down the slide and correspondingly, the bolt. No modification to the receiver or the BAR magazine was required.
April 10, 2002, 04:10 PM
I have no idea how fast they managed to get the Thompson in experimental versions. Production guns, however, were mostly around the 600 RPM rate.
I read somewhere they tried out a Super .38 version--maybe it was faster. Due to the long travel and heavy bolt system, I'd be surprised if they ever got any of 'em much over 800 RPM.
April 10, 2002, 10:02 PM
I'm only going on what I've read and been told, not what I've seen here. The cyclic rate of the Thompson M1928A1 was in the 650rpm range although variables shot it up to 750 or down to under 600 at times. The M1 Thompson was simplified by the removal of the Blish lock but the bolt was made slightly heavier to compensate for it. It was therefore about the same as the M1928A1 or maybe a bit faster.
The real difference came with the M1A1 which had the firing pin replaced with a nub on the bolt face. Other simplifications and modifications made the gun open-bolt. This gun fired much faster as lock time was essentially zero. It is my understanding that these guns surpassed the 1000rpm mark. I've seen M1A1's firing and can attest that, if it ain't 1000rpm, it must be faster! Of course new guns might have been much slower without the parkerizing broken off of the working surfaces.
What is interesting is that the magazine could feed the rounds that fast. 45ACP is very fat and has to travel further than, say, 9mm does.
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