Bullet stability

[Tinybob]

What causes a bullet to shoot extremely well (0.3 MOA) when it's called "marginally stable" by the Berger calculator?

The rifle is Savage 112 manufacturered in October of 1991. So pre-accutrigger, bone stock gun in 223 rem. Savage says it has a 1 in 14 twist, it's a heavy varmint barrel at 26".

Loads are with four different powders so far, IMR4166, IMR4198, H322, and Varget.

Same bullet and seating depth:
60gr Sierra varminter at 2.250".

It's like magic!

 

[jmr40]

From Bergers website:

Your bullet is MARGINALLY STABLE.
Your bullet stability is marginal. You may shoot good groups under these conditions, but the BC of the bullet will not be optimized.

It is still very possible to shoot very accurate groups, but the BC's listed for that bullet are no longer accurate.

Read the next chart below when using their website. It will give you the listed BC, as well as the corrected BC. Something like this. BTW I just used some random numbers to get an example.

Your BC is being compromised by: 8%

 

[Bbear]

The 'marginally stabilized' will come into play more as you go longer. If your .3" MOA group was at 100 yards, you may not see the results of the stabilization until you get out to 300 or more. If all you plan on doing is shooting critters and paper/steel out to 100-200 yards, you should be ok.

 

[Walkalong]

A bullet is either stabilized or it isn't. If it is, you're fine, even if "marginal" which only means you have no wiggle room. How far out will they remain stabilized is another issue.

 

[Tinybob]

Those groups were at 100yds during load work up. I do intend to go out to maybe 500yds, so the difference will probably show up then.

I did try some 69gr SMK and couldn't keep those on paper at all.

 

[BigDog1955]

may want check the for arm stock and clear space to the barrel if it is to be floating, if not u may want to fill it with epoxy and try it again?? Just a thought?? harmonics will do some crazy things!!!

 

[Tinybob]

As far as I know the barrel is supposed to float and is floating at this time.

Update to original post: I changed the stock to a Choate Ultimate Varmint stock. I had the same result as in the original synthetic stock.

 

[ironworkerwill]

Sometimes it takes a "trigger" to upset the stability if the bullet. Like going transonic.

Some bullet runs aren't as long as advertised either.

Mostly the stability is only a factor at longer distance as Bbear already stated.

I use the Choate Ultimate Sniper on my LDR rig as well. It's a good heavy anchor. And allows LOTS of clearance even on a full bull barrel.

 

[Varminterror]

Bullet twist slows down in flight just the same as bullet velocity. Staying stable to 100yrds isn’t difficult if it was stable at the muzzle, get too far out, and that’s when you hear guys like me say phrases like, “the wheels come off.”

Berger’s “marginally stable” span is large. Something can be “marginally stable” and shoot incredibly well to 800-1000yrds, or something else may be “marginally stable” and only shoot well to 100-200yrds. Play with their calculator for a little while, do your own small scale sensitivity analysis and you can see how much forgiveness you have in your stability factor.

Most of my long range specialty pistol loads are “marginally stable,” and most of them will run to 1,000yrds. I tend to be happy with anything over 1.1-1.2 on their scale, and haven’t found many 1.05-1.1 loads to hold up past 300.

 

[Tinybob]

I've noticed that Sierra's 55gr HPs and SPs are not as accurate as the 60gr HPs to 100yds in this rifle.

The 55s I've tried are 0.1" and 0.15" shorter than the 60s on average. I use a dummy round to figure out how far mag length is from the lands when seating. Actual mag length is 2.300". Jammed length on the 60s is 2.855" Jammed length on the 55s is roughly the same.

Is the difference in accuracy possibly the 55s are not settled down at the close range?