Understabilized bullets: What are the signs?

[Slowdive]

Other than keyholing, are there other signs that indicate stability issues (eg. group drift, spread groups, etc.)

Here is why I ask. I inherited a Rem 700 heavy barreled (.9" thick and 26" length) Win .243 that my dad bought in '81. Bought it as a varmint gun, but never shot more than 100 rounds thru it b/c it's just too heavy to carry around.

The twist is 1:10" RH. He also complained that he never got this gun to shoot better than 1.5" groups @ 100 yrds with factory ammo. IMO it easily should be a 0.5 MOA to 0.75 MOA rifle. I started in earnest to develop a load for it and finally got some 75 grain Sierras to group @ 0.6". This rifle is very OAL sensitive likes ~0.020" jump; I tried bumping the lands; I got 2.5" groups about 5" to the left of zero!

So I started experimenting w/ heavy bullets, 95 and 100 grain. A 1:10 should be able to stabilize these, but I'm getting about 3" of group drift and 1.25" to 1.5" groups. No keyholing, but my groups are falling apart.

For a RH twist rifle, which way (if any) would the groups move for an understabilized bullet?

 

[EddieCoyle]

Are you sure they're understabilized? What's the bullet length and velocity?

You can check here:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

ETA: Have you tried shooting them at a longer distance? One sign of a slightly understabilized bullet is better groups (MOA wise) at longer distances.

 

[Slowdive]

Are you sure they're understabilized? What's the bullet length and velocity?

You can check here:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

ETA: Have you tried shooting them at a longer distance? One sign of a slightly understabilized bullet is better groups (MOA wise) at longer distances.

Definitely not sure, I'm just stumped b/c a 1:10" .243 should be able to stabilize a 95 gn bullet but I'm not having great luck getting a sub-MOA group out of them.

I'm working off memory but the length is 1.1" the MV is ~2800fps. The Miller stability factor per the JBM calculator is ~1.2, which is borderline. I could push them faster, but I'd be at max pressure and the factor doesn't get much better. (@ 3300fps the factor is only 1.273).

 

[Jasper]

could be a LARGE number of things.

first and foremost. don't try to jam/kiss the lands with sierra matchkings. they're VERY jump-friendly. I load my .22cal 80gr'ers ~ .025" off the lands, and my .308 155's .039" off the lands (seemed to be a sweet spot). the 155's, as a reference, barely hold 1" @ 100yds....but they'll hold 2.5" (of elevation) @ 500yards. try benching the gun a bit farther.

another thought (don't take offense to it)...have you had anyone else bench the gun to test their results? i've learned recently shooting REAL tight groups off a bench is a bit of an art form, and does take a bit of know-how to do well. the spreads in groups MIGHT be you.

also, you SURE there's less than 100rounds through it? .243 is a barrel burner....and depending on usage, groups WILL open up after some time.

If you find yourself up near the north shore, shoot me a PM. we can take a look at it through my borescope...might be something as simple as needing a good cleaning.

 

[Slowdive]

..first and foremost. don't try to jam/kiss the lands with sierra matchkings. they're VERY jump-friendly.

Yes, I learned this the hard way. Never had any luck w/ touching lands. I once heard that .22's like to jump and .30's like to touch and everything in the middle is a guess....but I've never had good luck following this philosophy. Getting the right jump in my experience is more important than getting the charge weight right.

Another thought (don't take offense to it)...have you had anyone else bench the gun to test their results? i've learned recently shooting REAL tight groups off a bench is a bit of an art form, and does take a bit of know-how to do well. the spreads in groups MIGHT be you.

No offense taken, I know that I'm not capable of shooting better than 0.5 MOA off the bench even on a good day. On an average day I'm a 0.75" shooter w/ near perfect ammo, rifle, weather. I usually try to shoot lots of groups and at least two rifles in the mix with some known good loads to eliminate me from the equation as much as possible. In this case I did see a clear pattern alternating between the 75's and the 95's. I had a good day w/ the 75's ... 4 groups between 0.6" and 0.7 and one around 1". My best w/the 95's was 1.4". There was also a clear left to right drift, but that's probably barrel harmonics.

Shooting from the bench is an art form, I agree.

also, you SURE there's less than 100rounds through it? .243 is a barrel burner....and depending on usage, groups WILL open up after some time. If you find yourself up near the north shore, shoot me a PM. we can take a look at it through my borescope...might be something as simple as needing a good cleaning.

Thanks for the offer! A good inspection may reveal a lot, I'll let you know if I'm up that way. In terms of round count, I have to rely on my dad's memory. Since taking ownership I've put 200 or 300 thru. He is the original owner and quickly switched over to his much lighter 22-250 for varminting.

 

[DukeInFlorida]

Read Kicker's post regarding how he worked up a group for his Mosin.

You'll need to work up a load for your gun.

Accuracy is more than velocity. It's TUNING the load for the barrel.....

I recommend 2 grain jumps, starting with the starting load, and continuing to the max load, checking for signs of pressure along the way (stopping when you get to too much pressure).
Document the accuracy with each 2 grain load change.

Find the load that gave you the best result from that effort, and then work from 1 grain less than that to one grain over that, in .2 grain levels, until you find the MOST accurate load for your barrel.

Kicker wasn't a believer either.... until he tried it.

It's called, "Working up a load" and it is one of the most important parts of finding the sweet spot for your barrel. It truly is a LOT to do with barrel harmonics.

Just try it, and report back.

Other than keyholing, are there other signs that indicate stability issues (eg. group drift, spread groups, etc.)

Here is why I ask. I inherited a Rem 700 heavy barreled (.9" thick and 26" length) Win .243 that my dad bought in '81. Bought it as a varmint gun, but never shot more than 100 rounds thru it b/c it's just too heavy to carry around.

The twist is 1:10" RH. He also complained that he never got this gun to shoot better than 1.5" groups @ 100 yrds with factory ammo. IMO it easily should be a 0.5 MOA to 0.75 MOA rifle. I started in earnest to develop a load for it and finally got some 75 grain Sierras to group @ 0.6". This rifle is very OAL sensitive likes ~0.020" jump; I tried bumping the lands; I got 2.5" groups about 5" to the left of zero!

So I started experimenting w/ heavy bullets, 95 and 100 grain. A 1:10 should be able to stabilize these, but I'm getting about 3" of group drift and 1.25" to 1.5" groups. No keyholing, but my groups are falling apart.

For a RH twist rifle, which way (if any) would the groups move for an understabilized bullet?

 

[Bob J]

I use a slightly different approach for load development for long range rifles.... The first part is is similar to duke's but with the COAL adjusted to give me .010 jam.... Loads are then carefully worked up using TresMon's excellent 5 part tutorial on Snipers Hide with close watch for signs of overpressure...

A second ladder is then built and shot with that load and different jumps..... The first ladder gives you the powder load and the second gives you the optimal jump for your gun and that load..... The key sections are here:

Hand Loading for Long Range 3: finding start OAL

Hand Loading for Long Range 4: Powderin' it

 

[Slowdive]

Thanks for the input, I think I maybe gave too much info in an effort to provide context.

I should have simply asked the question hypothetically in this way... Load development aside, IF bullet stabilization is really the culprit, are there any visible symptoms that clearly identify it.

 

[EddieCoyle]

Thanks for the input, I think I maybe gave too much info in an effort to provide context.

I should have simply asked the question hypothetically in this way... Load development aside, IF bullet stabilization is really the culprit, are there any visible symptoms that clearly identify it.

I shoot Hornady.224" 75 and 80gr AMax's in a 1:8 barrel. At some velocities, the bullets are barely stabilized. The groups are worse than expected at 100 yards, but 'tighten up' at 300. The bullets actually get more stable the farther they travel. I think this is because they lose forward velocity faster than they lose spin.

If you can, try the same ammo at a longer distance and see what happens.

 

[bpm990d]

If they group at 100 yards, then they are stable. The accuracy may be bad, but they are stable. If it's a pattern then they are unstable. The 1/12 twist in the M16A1 is just not enough to stabilize the 62gr M855 ammo in the picture below. Those are 19" silhouette targets and those groups are huge.

EC is correct that bullets that are stable at the muzzle, will continue to increase their stability as they fly downrange because the forces that want to destabilize them, decrease much faster than the stabilizing spin.

 

[Slowdive]

Thank you! This makes sense, it's clear I need to execute a longer distance test to know if I'm borderline (per EC's example). With true instability, the groups go to pot in a hurry (per above). I'm not seeing that in my example.

 

[mike93lx]

I would not assume that a factory Rem 700 is a ".5 to .75 MOA" rifle. While it is certainly capable of it, the rifle will more than likely need work to achieve that.

If it was easy to hit that level of accuracy, people wouldn't spend as much as they do to just get down to .5 MOA

 

[Jasper]

I would not assume that a factory Rem 700 is a ".5 to .75 MOA" rifle. While it is certainly capable of it, the rifle will more than likely need work to achieve that.

If it was easy to hit that level of accuracy, people wouldn't spend as much as they do to just get down to .5 MOA

depends on the barrel, and quality of ammo.

my "benchmark" on my 700 Police (with federal gold medal match 168s) was 3/8MOA average (200yds, 4shots went under 1/2", the "flier" (which i yanked the trigger on) widened the group to 1.12" CTC (JUST over 1/2MOA)

never said it's "easy". a piss poor shooter can make the best of rifles group like hell.