Short barrels and fast powders

Posted by on April 1, 2006

Much of what I post to this forum is theoretical “what if ” stuff that probably has no practical usefulness. But one never knows and it is all intended to provoke further discussion.I have been thinking about light loads, ie pistol powders and cast bullets in rifles. We all know that there is a relationship between burning rate, pressure curve, bullet weight and barrel length. That is why MS200 will produce more velocity with a light bullet in a two inch barrel than MP200, but the opposite is true for a heavier bullet in a six inch barrel.This phenomenon is much more apparent in rifles because of their much longer barrels. I shoot a 180 grain cast bullet from my Sako in 308 Win. I load with 11 grains of MP200. While I haven’t pushed the limit, I reckon 12 grains is the maximum. That’s because the fast burning rate and resultant pressure rise will burst the barrel long before the bullet reaches the muzzle if the powder charge is not kept within limits. It also means that maximum velocity is reached in the first part of the barrel, probably within the first ten inches. Velocity in my rifle with 11 grains of MP200 is about 1600FPS.So far so good. More velocity needs a slower burning powder with a slower pressure rise, to accelerate the bullet for the full length of the barrel. In the 308 Win that used to be the medium burning MR200. These days it is S335. But a powder with a burning rate between those extremes will produce a MV also somewhere in the middle, right? Right. Whilst I’m not familiar with all Somchem’s powders, there are some, maybe S265, that are slower than MP200 and will thus produce higher velocity say around 2000FPS with about 20 grains of powder, but are still too fast to produce maximum velocity for calibre.There are two threads here. The first is that the faster powders are more economical at the cost of velocity. This would be relevant in a survival situation in which powder supply is limited. There is a lot of game that can be taken with a 180 grain bullet at 1600FPS, and not much soft skinned game that can’t be taken at 2000FPS. 11 Grains of MP200 is 700 shots per can of powder, 20 grains of S265 is 385 shots, and 42 grains of S335 is only 183 shots.Smaller powder charges and lower muzzle velocity is less muzzle report. My 308 cast load is a lot louder than 22 rimfire but very much less than a full service load, and therefore much easier to suppress if necessary.The second thread is to look at it the other way round. That is, if max MV of 1600FPS with MP200 is reached in say ten inches, why not fire it from a ten inch barrel? I have first hand knowledge of a silhouette pistol in 308 Win that delivers 2600FPS with a 150 grain bullet from a 15 inch barrel. That’s only 200FPS less than a full length rifle barrel.This makes possible practical ballistics in short barrels. So the logical answer for those who might need a light, portable and easily concealable weapon would be a silhouette type pistol. But it also makes for a very compact rifle. The clue lies in a report I read some time ago which described the result of cutting a 22RF barrel to ten inches. The rifle was naturally then out of balance. The solution was to extend the barrel with a ten inch tube not much bigger in diameter than the barrel. There was no reduction in accuracy but muzzle report was reduced because the tube was in effect a tunnel.What has that to do with compact rifles? A fifteen inch barrel with a detachable tube or possibly a suppressor and a folding stock would be a very portable and compact package, with a MV capability from 1600 to 2600FPS depending on the powder used. Might also be a handy combat piece in some circumstances.This concept is not that much different from the De Lisle carbine we discussed a while ago, but the ballistics are much superior.[Originally posted to SATalkGuns -- Admin]

Comments

Respond | Trackback

  1. G. du Preez June 28, 2010 3:08 pm

    Is S321 better than S335 in a 30-30 Win. barrel?

  2. Richard July 19, 2011 5:25 pm

    In reply to G du Preez. S335 is generally considered the best all round powder for the 30-30. S335 is very similar to 3031 which is the standard in the US. S335 and 3031 are of medium burning rate, designed for cartridges with powder capacities not too big in relation to calibre/bullet weight. It is the preferred powder for the 308 Win. The 30-06 with a bigger case but same bore diameter, works well enough with 335, but better with slightly slower powder, especially with the heavier bullet weights. 335 Also works well with big calibre straight cases like the 458 Win.

    It might be expected that the 30-30, being a smaller case than the 308 Win, would do better with a slightly faster powder. But the standard bullet weight of 170 grains favours a slower powder, so the two factors balance out to S335.

    321 Is a bit too fast, in my opinion. Interestingly, my friend Wouter de Waal, who shoots the 30-30 quite a lot, has discovered that his rifle shoots the 170 grain bullet best with S335, exactly as expected, but shoots the 150 grain bullet better with S341.

    So it shoots a lighter bullet better with slower powder, opposite to what would be expected. But that is a peculiarity of a particular rifle, and not to be taken as normal. Usually, in any given calibre, heavy bullet = slower powder, and vise versa. It is therefore possible that the 30-30 might shoot a lighter than standard bullet well with S321, but don’t try it without asking Somchem’s advice. Generally, S335 is the right powder, at least from 150-170 grain bullets.

    The relationship between powder burning rate, case capacity, bore diameter and bullet weight is a complex topic of its own. For example, although the 243 Win has an identical case with the 308 Win (except for the neck, obviously) it likes slow burning powders of the sort used for magnum calibres. That shouldn’t be surprising, as the bore is much smaller in sectional area, a characteristic that it shares with calibres like the 7mm Rem Mag and 300 Win Mag.

Comments

Comments: