It is why certain powders also work in a Garand to cycle it and others will not. While this does happen in milliseconds, it can have an effect on a gas operated weapon like the carbine. What I look at is the Pressure Curve of the powder as the bullet leaves the case and some slower powders have a a longer curve that develops the same velocity in a longer barrel with slightly more or slightly less overall CUP pressure depending on the powder. It's an effect of the expansion ratio again.ĪA7 is faster than 2400 which is faster than 110 /296/Imr4227 in that order. The 30-carbine is more like the 357 & 44 Mags when shot from leverguns: most of the velocity is built up in the first 12-14", and the cases are just too small to get enough powder into to take advantage of any additional bbl length. When you say longer barrels need slower powder in most cases, you are right, except here we are working with one of the exceptions. Even so, when taking into consideration the very large expansion ratio to bullet weight, a faster-still powder will be necessary. AA#9 is almost identical in burning speed to 2400, but because it has very fine ball granules, more of it will fit in the same space an important consideration with the tiny 30-carbine case. The reason 2400 is not a good choice is because in addition to being too slow, it is a very bulky powder with big granules. As the sabot/bullet combo move down the bore, the chamber is constantly getting bigger, therefore pressure is dropping precipitously, hence the need for a faster burning powder than the typical 30-carbinepowders. This in effect, drastically reduces pressures. 308" bore the gas is pushing down, so there is no constriction or funneling of the gas. With a 30-carbine firing a sabot, there is still a. 224" bore which all the gas must squeeze down. The difference between the Spitfire and a carbine loaded with a sabot round is the expansion ratio. It is one of the few (generally considered slow/magnum pistol powders) powders which are in the narrow band on a burn-rate chart which give good results in the 30 Carbine (from about AA#9 through, arguably 680/1680, which many would argue is too slow). It is faster than H110, but only a very small amount. Longer barrels need slower powder to produce optimum results in Most cases.Ī lighter recoil spring may help with the cycling problem too.Ģ400 is not what I would consider a "fast" powder, because I am looking atthe entire burn-rate chart from Bullseye to 5010. The Spitfire ran 2650 to 2800 FPS mv with IMR 4227, where a normal Mil spec 30 carbine runs ~ 1950 FPS or less.
If its NOT working with 2400 which is a fast powder why wouldnt you go slower powder like was proven in the Johnson Spitfire to work much better in both cycling, velocity, and accuracy in the Carbine action ?