Does meplat size affect stability?

[26Charlie]

This question relates to plain-base bullets of similar weight. All groups referred to below are shot at 100 yards; the .45-70 bullets are .457 and the .25-20 bullets are sized .259. The .45-70 has a 1/8" card wad in contact with the base of the bullet. No fillers were used.

I have two instances to relate, then invite comment:

Using 457193, a 400 gr. PB bullet in two .45-70 Marlins, I got good accuracy in both guns with 35 gr. of Scot 4197 (like IMR 4198).
I picked up a mould for 457643, a 400 gr. PB but with a meplat half again as wide - which translates to two and a quarter times the frontal area. Using the same load, one Marlin gave 10" groups with tipping of bullets, the other gave minute-of-tractor-trailer groups, throwing them broadside and into the dirt everywhere. I backed the load down 10% to 31.5 gr. 4197, and the one Marlin now gave me a 4" group with a flier, the other gives me a vertically strung 15" group, with tipping. But there is a lot of unburned powder with this load, so I don't want to reduce it further.

Using 257283, an 87 gr. PB bullet in two .25-20's, one a Marlin lever action and the other a Savage 23B bolt action, I got good accuracy with 9.5 gr. of 4227. I picked up a mould for RCBS 25-85-FP, which is a PB bullet casting at 83 gr., a little shorter than 257283 but again with a meplat half again as broad. The groups I got were about 10", all showing at least from 12 to 30 degrees of yaw on the bullet - elongated holes. Mindful of the .45-70 experience, I backed the load down to 8.5 gr. of 4227. The Marlin now shows 3 out of 10 fliers, the Savage 2 out of 10, the rest of the shots grouping as well as the 257283 bullet but with 2 to 20 degrees of tipping showing.

So the question is, to what to attribute these results? I can't think of but one significant difference, the larger meplat diameter, hence greater frontal area, causing some sort of aerodynamic imbalance to the bullet that the other similar bullet doesn't suffer from.

[Leftoverdj]

The extreme case is the .38 Special WC w/ 100% meplat. Inch groups at 50 yards, lucky to get 12" groups at 100. In the same rifle, the Lee 356-153-2R with no meplat at the same velocity gives 1.25" at 50 and 2.5-3" at 100.

Gotta be the meplat, or, more precisely, the terminal velocity difference caused by the large meplat.

[felix]

26Charlie, anything that increases air resistance demands more twist. Meplat size is a major culprit. See if there is a seeable contrast in a clean versus dirty barrel. Your grooves might not be deep enough, for example. ... felix

[The Nyack Kid]

try more velocity .
the wad cutters are more accurate at 50 yard than 100 because they havnt lost their velocity yet . wide nosed boolits shed speed like crazy they have to start out fast to stablize at farther ranges . Lbt style boolit are notoriouse for this . i find best accuratcy with my WFN style boolits at the higher end of the scale.

[David R]

I didn't see any velocities for those loads, but do they go near the speed on sound? If they cross the sound barrier, it can raise havoc with them.

Just a guess.

David

[Bass Ackward]

So the question is, to what to attribute these results? I can't think of but one significant difference, the larger meplat diameter, hence greater frontal area, causing some sort of aerodynamic imbalance to the bullet that the other similar bullet doesn't suffer from.

Charlie,

This isn't rocket science. If you had a varmit rifle and you wanted top accuracy and long range performance. Would you choose a flat nose? A round nose? Or a spitzer? Same, same.

The real question is where is the cut off? Well, this is hard to say because there are so many variables to bullet fit. But when I tried to control all of these to the best of my ability, I found @ 75% to be the cut off for loads that produce close to 2000 fps muzzle. Smaller meppys if you want reduced loads / velocities to produce top accuracy. But I have never had trouble out to 200 yards when I maintained the disapline to hold 60% as long as I had the bearing length to handle it. But that is bore diameter dependent. If you cut bullet weight, you get a shorter bullet. So naturally a worse ballistic coefficients, so lighter bullets get smaller meplats yet. Most of my 45s spout 50% meplats and I haven't found a range they crap yet for hunting.

In 45 caliber, 2X bore diameter bearing length occurs at around 420 grains if you hold unsupported nose weight to 20% of bullet length and maintain a 60% meppy. So in short, I never go over 60% meplats on my rifle bullets. If I need more than that, I go to a bigger caliber or I step on it.

But I don't think that is your problem in this case. I would bet it is ignition. That's why the 13 grains of Unique is so popular in that PB range. I can't tell you about the 25-20, but would bet you are fighting the same enemy.

[Bret4207]

I'll disagree with BA a little and point out that when you come up against a new problem, it may as well be rocket science till you get some help. That being said, I would agree that it's probably more than just metplat size. That RCBS cowbot mould has a pretty good rep for shooting nicely, so it could just be the powder/bullet/gun/ignition/velocity/rifling combo. Off the top of my head I'd check bullet diameter too, because the RCBS has a much shorter beariing length IIRC and sometimes you need a slightly fatter bullet to help get good ignition and rifling "grip". IOW, you're going to have to play around with it a bit. Faster, slower, heavier crimp, different powder.

On another subject BA mentions: there was a pretty lengthy experiment done some time ago by the Gov't I think comparing RN to SP accuracy-wise. No difference in those tests. But, the SP ranges better. If I'm choosing a varmint bullet I'l choose the cheapest most accurate one, not necessarily SP.

[beagle]

I've had really good results with the 457463 in a ballard rifled .45/70 Marlin. Same story with the 457193. Now, i'm sizing .459 and that seems to give much better accuracy than bullets sized smaller. One of my shooting partners has had similar results in a Browning BPCR in .45/70.

The .25-20 Marlins with the 1-14 twist did not shoot either of those bullets well. They seem to be too long for the twist and the velocity normally associated with the .25-20. I finally upped the velocities to 1800 something and got about 1 1/2" accuracy with the 257312 at 100 yards but I was still showing elongated holes in that group. The 257420 seems to be about the heaviest the Marlin 25-20s will handle. I rebarrelled that rascal with a 1-12 twist and the elongation went away.

I did notice a difference in accuracy between some 300 grain LBTs and the RCBS 44-300-SWC launched at the same velocities out of my Super Blackhawk. This I attributed to the LBT wide meplat at 100 yards./beagle


This question relates to plain-base bullets of similar weight. All groups referred to below are shot at 100 yards; the .45-70 bullets are .457 and the .25-20 bullets are sized .259. The .45-70 has a 1/8" card wad in contact with the base of the bullet. No fillers were used.

I have two instances to relate, then invite comment:

Using 457193, a 400 gr. PB bullet in two .45-70 Marlins, I got good accuracy in both guns with 35 gr. of Scot 4197 (like IMR 4198).
I picked up a mould for 457643, a 400 gr. PB but with a meplat half again as wide - which translates to two and a quarter times the frontal area. Using the same load, one Marlin gave 10" groups with tipping of bullets, the other gave minute-of-tractor-trailer groups, throwing them broadside and into the dirt everywhere. I backed the load down 10% to 31.5 gr. 4197, and the one Marlin now gave me a 4" group with a flier, the other gives me a vertically strung 15" group, with tipping. But there is a lot of unburned powder with this load, so I don't want to reduce it further.

Using 257283, an 87 gr. PB bullet in two .25-20's, one a Marlin lever action and the other a Savage 23B bolt action, I got good accuracy with 9.5 gr. of 4227. I picked up a mould for RCBS 25-85-FP, which is a PB bullet casting at 83 gr., a little shorter than 257283 but again with a meplat half again as broad. The groups I got were about 10", all showing at least from 12 to 30 degrees of yaw on the bullet - elongated holes. Mindful of the .45-70 experience, I backed the load down to 8.5 gr. of 4227. The Marlin now shows 3 out of 10 fliers, the Savage 2 out of 10, the rest of the shots grouping as well as the 257283 bullet but with 2 to 20 degrees of tipping showing.

So the question is, to what to attribute these results? I can't think of but one significant difference, the larger meplat diameter, hence greater frontal area, causing some sort of aerodynamic imbalance to the bullet that the other similar bullet doesn't suffer from.