243 short barrel powder

[Dannywayoflife]

Hi guys and gals,
I've finally placed my order for a new 243 barrel for my dta srs covert. I have gone with a lowther 1in7" twist @18" in length. I will be throwing 2 bullets the 105 Amax and the 115 dtac. Can anyone recommend me a powder that will suit? From what I've read I wants slightly faster then normal powder so as to fully burn in the barrel. I have thought about the following

Ramshot hunter
VV N160
VV N560
H4350
Rl17

I would like to achieve 2700 with the 115's and slightly more with the 105's.

Cheers guys

Danny

[Mr_Logic]

Why have you gone for such a short barrel for heavy bullets? Using N160 I was hitting 29 or so with a 105 Amax.

IMR7828 SSC gave good speeds but accuracy was lacking, with 95Noslers and 108 Bergers. I have an 8-twist border barrel.

[Sim G]

This idea needs some constraints. It would be more precise to say a powder that produces the most velocity in a long barrel at a given peak pressure value will be fastest in all barrel lengths until the bore length the bullet travels in gets pretty short. So where Rocky says "regardless of barrel length", there is a lower limit, but you don't ordinarily run up against it. I don't expect to see that rule broken within the 16" lower limit for rifles that don't require special short barrel permits. In handguns, which use faster powders in the first place, you see 2" snubbies violate the rule with some powders, but I don't recall seeing a 3" barrel do it.

Velocity depends on the average pressure behind the bullet while it is in the bore. In general, a slow powder has lower average pressure in the first inch or two of bullet travel than a faster one does because of its slower pressure rise. But once you pass the pressure peak, the slow powder maintains higher average pressure behind the bullet, which makes up for the slower rise if there is enough barrel length left to allow it to do so. In other words, the only way a load that produces the most velocity in a long barrel can be outperformed by a faster powder is if the bore is made so short that a the slow powder's higher average post-peak pressure fails to make up for its lower average pressure in the first inch or two of bullet travel.

It's tough to find out at what barrel length that crossover happens from information in a load manual. A quick look through, say, Hodgdon's online load data, or at any other load data that includes measured pressures shows the maximum loads have quite a variety of pressure values. That makes an apples-to-apples comparison tough, since it only works out for same value pressure peaks. I'm not sure why those pressures vary so much and are all called maximum. It has to be an artifact of how they go about the load development process.

I put the .243 in QuickLOAD and ran a powder table of constant 60,000 psi peak pressure (SAAMI maximum) with the Hornady 100 grain BTSP seated to 2.710" in a 24" barrel and using a default maximum load density of 105% (medium compression). QuickLOAD's not a perfect model, but for comparative purposes like this it is pretty good. It shows the same powder producing maximum velocity in barrels from 24" (about 21.5" of actual bore) down to about 10" (about 8.5" of bore). That's Norma MRP which is pretty slow for .243 and would, in real life, probably not be the best for ignition consistency. It gradually is displaced by Reloader 17, which is slower than medium powders, but not very slow.

An interesting thing is that nothing in the medium burn rate range ever puts in a serious appearance. When you get the barrel down to 7" (about 4.5" actual bore length), the highest velocities come from Reloader 17 and Accurate 5744, which is much faster than Reloader 17. The 5744 case fill is poor, at about 65%, but it takes that kind of pressure peak speed to get the average pressure up enough to beat Reloader 17's pressure advantage after the peak. Other top choices then include the two 4198's. By the time you get to 5.5" of barrel (about 3" of bore), Reloader 17 has dropped down and it's all about fast powders. 5744, 4198, 4227. Note too, though, that in such a short barrel the ranking has a lot less meaning as a wide range of powders are all withing 20 fps of one another.

Note, too, that, while interesting in theory, nobody would run a 60,000 psi round with such very short bore lengths except for very short range. The old Frank Brownell U of M study from 1965 tried that experiment and found recovered bullets badly distorted at the base by the high pressure muzzle blast. Their behavior would not be very predictable.

[dromia]

The old Frank Brownell U of M study from 1965 tried that experiment and found recovered bullets badly distorted at the base by the high pressure muzzle blast. Their behavior would not be very predictable.

Remember readers, just because it is old doesn't mean it is bereft of value as some on here would have you think.

A recommended read to all student of ballistics and hand loading.

[Dannywayoflife]

Why have you gone for such a short barrel for heavy bullets? Using N160 I was hitting 29 or so with a 105 Amax.

IMR7828 SSC gave good speeds but accuracy was lacking, with 95Noslers and 108 Bergers. I have an 8-twist border barrel.

The covert is set up for short barrels. I've found with the 308 16" barrel that heavy bullets seem to loose far less speed in short barrels than light bullets do. I can get the 208 Amax going at 2350 in a 16" barrel without hitting pressure.

I've also found that shorter barrels seem to work better with a slightly faster then normal powder. I think I'm going to settle with ramshot hunter as I love ramshot powders

[Dannywayoflife]

This idea needs some constraints. It would be more precise to say a powder that produces the most velocity in a long barrel at a given peak pressure value will be fastest in all barrel lengths until the bore length the bullet travels in gets pretty short. So where Rocky says "regardless of barrel length", there is a lower limit, but you don't ordinarily run up against it. I don't expect to see that rule broken within the 16" lower limit for rifles that don't require special short barrel permits. In handguns, which use faster powders in the first place, you see 2" snubbies violate the rule with some powders, but I don't recall seeing a 3" barrel do it.

Velocity depends on the average pressure behind the bullet while it is in the bore. In general, a slow powder has lower average pressure in the first inch or two of bullet travel than a faster one does because of its slower pressure rise. But once you pass the pressure peak, the slow powder maintains higher average pressure behind the bullet, which makes up for the slower rise if there is enough barrel length left to allow it to do so. In other words, the only way a load that produces the most velocity in a long barrel can be outperformed by a faster powder is if the bore is made so short that a the slow powder's higher average post-peak pressure fails to make up for its lower average pressure in the first inch or two of bullet travel.

It's tough to find out at what barrel length that crossover happens from information in a load manual. A quick look through, say, Hodgdon's online load data, or at any other load data that includes measured pressures shows the maximum loads have quite a variety of pressure values. That makes an apples-to-apples comparison tough, since it only works out for same value pressure peaks. I'm not sure why those pressures vary so much and are all called maximum. It has to be an artifact of how they go about the load development process.

I put the .243 in QuickLOAD and ran a powder table of constant 60,000 psi peak pressure (SAAMI maximum) with the Hornady 100 grain BTSP seated to 2.710" in a 24" barrel and using a default maximum load density of 105% (medium compression). QuickLOAD's not a perfect model, but for comparative purposes like this it is pretty good. It shows the same powder producing maximum velocity in barrels from 24" (about 21.5" of actual bore) down to about 10" (about 8.5" of bore). That's Norma MRP which is pretty slow for .243 and would, in real life, probably not be the best for ignition consistency. It gradually is displaced by Reloader 17, which is slower than medium powders, but not very slow.

An interesting thing is that nothing in the medium burn rate range ever puts in a serious appearance. When you get the barrel down to 7" (about 4.5" actual bore length), the highest velocities come from Reloader 17 and Accurate 5744, which is much faster than Reloader 17. The 5744 case fill is poor, at about 65%, but it takes that kind of pressure peak speed to get the average pressure up enough to beat Reloader 17's pressure advantage after the peak. Other top choices then include the two 4198's. By the time you get to 5.5" of barrel (about 3" of bore), Reloader 17 has dropped down and it's all about fast powders. 5744, 4198, 4227. Note too, though, that in such a short barrel the ranking has a lot less meaning as a wide range of powders are all withing 20 fps of one another.

Note, too, that, while interesting in theory, nobody would run a 60,000 psi round with such very short bore lengths except for very short range. The old Frank Brownell U of M study from 1965 tried that experiment and found recovered bullets badly distorted at the base by the high pressure muzzle blast. Their behavior would not be very predictable.

I have decided to go with ramshot magnum as my first trial powder. QL seems to indicate this is the fastest in terms of fps of those that I had plugged into ql. I should also get a full case with this powder as well which is nice. According to ql it will be about 97>% burnt in the 18" barrel also.

[Sim G]

QL is purely a theoretical programme, it should never be used in lieu of a decent loading manual. QL themselves go to great lengths to point this out. I always found QL somewhat lack lustre in the actual performance, but it is a good theoretical jump off point and a great toy!

Good luck with your search. Have you a decnt chrony?

[Dannywayoflife]

We have a magneto at my club mate :) that should do. I would like to buy my own chrono too but can't make my mind up between a superchrono ( the accoustic one) or a magneto.

The ramshot data does indicate that magnum would give very good speeds with a 24" barrel so it's fair to assume that mine will be less than that.

[Sim G]

Guns & Ammo years ago did an experiment that took a couple or three calibers and using factory ammo, chrono'd the loads from a 24" barrel then cut an inch off each time rechronoing. If memory serves, depending on calibre, it was between 50 and 70 fps loss for every inch of barrel cut. They only went down to 16 inches.

I'd be interested to know what your attained velocity is and would it be between the 300 to 420 fps slower than loading data for a 24" barrel.

[Dannywayoflife]

I will let you know mate once loads are developed