Diagnosing a bullet strike

[Dabbler]

Next step on the basics: what are the differences in behavior with and without the compensator. Have you checked your headspace? What is your bullet jump to the lands and grooves (just to make sure it isn't excessive)...

It would be far easier to hold the rifle in hand and see everything! Sorry for the tedious questions, I'm just working through a diagnostic tree to create some ideas...

In many years of playing with rifles I've seen only one barrel we couldn't make shoot well. It completely checked out, borescoped it, rechambered it several times for different cases, etc. That was once in perhaps 50 barrels. it is *possible* but I wouldn't go there just yet.

 

[cdhknives]

I appreciate the help. I am at my wits end too. The rifling is a bit light but I still can't see the bullets skidding. The barrel is plenty tight in the receiver. I can feel no indication of a rough or tight spot in the bore. I've changed scopes and loaded some different weights...need time for another range trip. Everything is being shot with a bare muzzle from now on to eliminate variables.

 

[seasicksteve]

Quick recap you have tried with and without the muzzle devices? Have you tried a bullet other than the nosler at subsonic velocity?Do you have access to a borescope? I am wondering if the gas port might have a burr or some severe bore damage at the port that is damaging the bullet jacket as it passes the port. Just a spitball

 

[Dabbler]

doing my back-of-the-envelope calculation, the 220gr would need higher muzzle velocity to get stabilized. For chuckles, see if you can buy 10 or so shorter bullets, along the lines of the 190gr VLD berger hunting bullet. They say that 1-11 twist is all you need, but that is assuming transonic speeds at 2000 fps and above. At 900 fps, 1-7 seems about right.

- I'm suspecting that if you need to be subsonic, you need to use an easier to stabilize bullet.

How much can you pressure up the cartridge savely: Can you go 1300fps in the nosler 220's? If you can, and do, and they stabalize, then you need to rejigger your subsonic system.
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Now for another possibility... When I custom loaded some heavy .308 Winchester subsonic cartridges, we had to go 'Bulseye' pistol powder, because we got way too much muzzle blast for our compensators to reduce. With the ultra fast burning powder, our muzzle pressure at the moment of the bullet leaving the barrel was much less.... BTW without the compensators, we couldn't get the bullets to stabalize at all.
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That's about all I've got for you. Good luck. Please let me know if you solve it!

 

[cdhknives]

Simplest explanation and all that...and as an Engineer I hate to say this...but the twist formula appears to have failed. I refused to believe it might mislead me, assumed this had to be a hardware problem, but:

Replaced scope. Bare muzzle (no brake, suppressor, flash hider, etc.)
178 grain Amax, full power @ about 1700 fps gives a 1.25" group with nice round holes...no surprises as it is a cheap nitrided cromoly barrel.
180 Nosler BT at about 900 fps gives a 3" group with some oblong holes...3 groups actually, 2 pairs and a triple (7 shots) roughly on the points of a 2.5"-3" triangle
220 Nosler match 900 fps gives a couple of oblong fliers as usual
240 grain Sierra Match bullets @ about 900 fps only hit the paper twice, one was sideways and about 8" away from the single round hole...with 3 misses. In the freaky things category, the sideways hit was right on top of the 178 grain Amax group.

The Miller twist formula (linked above on the Berger website) says the 220 grain bullet should be reliably stable down to 500 fps. Not happening...can't find anything mechanical...pretty much every troubleshooting thing I can try says stability so either physics is lying, I am missing something, or the twist formula is failing for this application.

https://imgur.com/a1tJ5jv

 

[cdhknives]

Now I will try 220 grain round nose bullets, but if it won't stabilize those I throw it away. If this won't shoot subsonic loads it is useless for it's purpose.

 

[Dabbler]

so I'm suggesting that stabalization depends on a lot of factors. For instance, 151gr Berger 6.5mm needs 1:7 twist... but it will only stabilize if I load to >2900fps. The equation shows that it will stablize at much slower speeds. Shooting shows it won't.

You can use the same setup with 'easier to stabilize' bullets. You might not get the same momentum, but it may be the price you pay for acuracy. Give the 190gr or lighter VLD bullet... It will stabilize easier (but you will have to change your load) Stabalization depends on RPM and the bullet profile, among a lot of other things. That's why we had to use Bullseye powder in our subsonic .308 winchester.

 

[Tozguy]

It seems to me that VLD and boat tail bullets are designed for high muzzle velocity long distance applications.
For subsonic accuracy at 100 yds it seems to me that flat based hollow points (even if not match grade) would stand a better chance of stabilizing with any barrel.

 

[pdentrem]

Same thing with 250-3000. The originals had 1:14 twist and will no stabilize 100 grain long nose spritzer bullets but will shoot 100 grain round nose just fine. The suggestion to go back to flat base is also valid but not what you are looking for.

 

[Dabbler]

I was just trying to encourage some experimentation with different bullet types, as we found that our expectations and the reality didn't match well. On our 308 we went with flat base 190 gr bullets, but we had to change the powders to get what we wanted. Isn't that the fun of shooting sports?