Question about twist rates and twist rate calculators

[Acronin243]

Long story short I built a 1:9 22-250AI. Yeah, yeah, yeah, 1:9 for this cartridge isn't ideal, but the .223 barrel I reamed out was cheap.

anyway, it shoots the 75gr ELDM's really well at decent velocities (3250fps), however when I start using the Berger twist rate calculator is it marginal at best and drops the BC of the bullet.

I completely understand all of this, but as I continued to use the twist rate calculator I realized that a 1:9 doesn't stabilize many of the "heavy" bullets. The 69gr TMK shoots outstanding, but again the twist rate calculator says it's only marginal, but Sierra even says a 1:9 should stabilize it.

My question is this:
If you find a load that the twist calculator says is only marginally stabilized, but your rifle shoots really well, do you keep that load and then adjust the BC in the ballistic calculator according to your shooting results (drop at 300, 500, 800, etc.)? For instance if the BC on a bullet is .398, your rifle shoots it very well, but your drops don't match to what the ballistic calculator has, so you lower your BC to say .372 in the ballistic calculator until the drops match the BC.

OR... do you just shoot a bullet it's stabilizing 100%?

I appreciate the help on this, because if it's not already obvious, I'm relatively new to this stuff.

 

[TheOfficeT-Rex]

If it makes round holes on paper, send it. True your software at the longest range you have convenient.

 

[Luckyshot]

Change the BC until it matches drop.

All that matters about BC is that it stays consistent. If it is, its just a number in your calculator with a predictable outcome. Check your groups at the furthest distance, and if bullets aren't keyholing, you're good.

Thats really all that "proper twist" does in the first place--it keeps your BC constant in all conditions

 

[Sig Marine]

If the rifle/bullet shoot well at the maximum yardage I want to shoot, I will use that load and calibrate the BC to match my dialed elevation AFTER I’ve calibrated by muzzle velocity out to 600-700 yards. Then I will calibrate the BC at 1000-1200 yards until numbers line up.

Unfortunately many shooters don’t look at twist rate in relation to bullet RPM using hard numbers. A shooter is best off to decide on the bullet and weight he wants to use and then purchase a barrel with the twist that offers the best RPM for performance at the speeds he wants to run. If you want to run fast, you have to slow the twist. Heavier bullets typically take a faster twist because you can’t run them as fast whereas lighter bullets running fast use slower twists

 

[Acronin243]

This is EXACTLY the answers I needed fellas. Thanks so much for the info.

I think I'm going to drop down a bit, use a combination of the RPM, factory intended minimum twist rate, and of course accuracy at a distance. I think this will make me a happy camper if I fall within those guidelines. Then adjust the BC in the ballistic calculator to match true results.

Thanks a lot!

Andy

 

[Ledzep]

Use Hornady 4DoF if your bullets are in the library. The program has a built in stability calculation that is using physical properties of whatever specific bullet you select, not a generalized Miller formula. The SG numbers it spits out are much closer to reality.

That said, SG of less than 1.0, don't waste your time. SG from 1.0-1.2, most likely going to encounter precision problems to some extent. Anything about 1.2 or higher usually shoots well, but the lower the SG, the more AoA and more drag. IMHO it's a small percent drag increase and not worth worrying too much about, but it's there.

There are some other things that can mess with results. Asymmetric gas flow out of the muzzle (think A2 flash hider or to a greater extent AK slant 'brake') can cause some pretty impressive initial AoA that can push the limits of dynamic stability that the bullet may or may not recover from.