My opinion on overstabilization

[fullersson]

In another thread glo asked anyone who had experience with overstabilization to start a thread about it. Here's my experience and opinion on overstabilization. Thanks, Brian Brown



I don't know if you would call bullet failure an overstabilization problem or not but it is/can be caused by too much twist rate. My first benchrest gun was a .243win 1in8" twist shooting 105 VLD Bergers. It shot very well all the way out to 1000 yards with no problems. One day I was doing some testing with the 88gr FB and I noticed I wasn't hitting the paper at 300 yards. I'm not bragging but hitting a piece of notebook paper at 300 really should never be a problem once you have been shooting long distance for a while. Anyway I had a buddy of mine watch me shoot and he noticed little puffs on the shots that I didn't hit the paper. I watched him shoot a few and noticed the same thing.

I'm not saying there wasn't another contributing factor such as a rough throat or some other problem with the barrel because by the time I was doing this testing the barrel had a lot of rounds on it. But IMO the condition of the bore or barrel in general wasn't the main problem because the 105's were shooting fine.

 

[bozo699]

Brian,
Sounds like the bullet was blowing up on you, to much speed and or twist maybe?
Wayne.

 

[deadlyswift]

Fullersson,
One shooter in a F class match was shooting 10s and Xs and then we had to call for a mark on his target. When the target came back out of the pits it was scored as a miss. Many of us were scratching our heads? Later on it happen again. In my spotting scope I seen the puffs that you are talking about. (Did the bullet blow up???) Here is the thing. This rifle was a .308win shooting 185 LRBTs in a 1in11 twist 30" bbl. I'm guessing things were more due to just a cpl of bad bullets in his lot more than too fast of a twist or too much speed. Being its a 308win and 185gr pill. Vel to me is unknown, but most likely somewhere around 2750fps + or - a few FPS would be a real good guess.

Great topic tho. What really goes on with a bullet in flight is something that I often wonder about?

 

[fullersson]

Wayne,

IMO it was a problem of too much velocity and too much twist rate. The 105's were pushing 3200+/- fps. I used the same load for the 88's. I'm not sure of the velocity but I would think it would have been somewhere in the neighborhood of 3500+/-. IMO that much velocity combined with that much twist combined with a barrel that was at the end of its life caused bullet failure.

Glo had asked for an opinion on overstabilization. IMO overstabilization can be as much of a problem as understabilization under the right conditions. Thanks, Brian Brown.

 

[KevinThomas]

Brian,

The problem here isn't overstabilization, per se, but a combination of a number of factors that gives the results yousaw. Bullet breakup can happen even with bullets that are properly stabilzed (relatively low SG) given a thinly jacketed bullet, a rough bore, eroded throat, or a number of other factors. we saw this at Perry two years ago during the Small Arms Firing School. Hornady provided their 75 grain Match ammo, and AMU provided the rifles. We had major problems with bullets blowing up, very obviously, some of them within 50 yards of the muzzles. They looked like little Flak bursts in some cases, and streaking trails of smoke in others. This was on Viale, where I was one of the instructors. Over on Rodriguez, the USMC was working with another group of students, same ammunition, but they provided the rifles. Not a bit of trouble over there. The AMU guns were very well worn (i.e., just about shot out) while the Marine guns were fairly new. This past year, Rock River provided guns for the class, and there was no problem at all, again using the Hornady ammunition. Just a combination of worn barrels, a fairly (but not excessively) thin jacket and the right velocity range did the trick. For what it's worth, I've seen this same problem in fairly new barrels when the rifling is still very sharp, and again once the throat becomes extremely eroded and rough. I've also seen it in rifles that were in good condition and which were delivering fairly moderate velocities. The worst example I've ever seen was a 6.5x55 that was delivering its bullets to the target with so much lead outgassing from the ruptured jacket, that it was actually cutting the target. There would be a bullet hole, with a very obvious "comet tail" emanating from it, so heavy that it literally cut the target clean through for a distance of a quarter of an inch or so away from the bullet hole. I would have loved to seen some HS photograpy of that bullet as it pierced the target. In any case, we're not talking about a true "over stabilization" (don't like using the term, since it's technically a misnomer) issue here, but another problem entirely. Berger's actually done some very fine work on this issue, since it was a problem they were having with some of their line. They investigated it, found the true causes of the phenomenon, and corrected it, much to their credit.

 

[WyleWD]

Temperature may play into this as well. IMHO.

Saw a fellow shooting a 22-284 one afternoon. He is a very accomplished and careful reloader/shooter. He had worked up the load and shot it several days and we watched him confirm his sight in that morning. By about 2pm he wanted to get a check on the wind drift affect on it at 400yds. About 3 out of 5 shots would produce a bushel basket sized puff of light gray "smoke" at about the 20 yard line followed by an odd tone, followed by a rancid smell. It was about 95* then.

Later that evening and the next morning, shot it again with the same loads and just fine. WD

 

[fullersson]

I understand that rough bores and thin jackets can contribute to bullet failure. What does everyone think about purely over spinning a bullet? Can a bullet be over spun to the point that it tears apart? Taking the rough throats and sharp rifling out of the question. Can shear RPM cause bullet failure? IMO yes it can but I've been wrong before. Thanks, Brian Brown.

 

[Eddie Harren]

By God, I think you are on to something here!

 

[KevinThomas]

Brian,

Everything has its limits, and RPM is no different. Most of the concern about "overstabilizing" bullets is really unfounded, at least where accuracy issues are concerned. Benchresters tend to worry about it most, but I hear the same questions about whether an "overstabilized" bullet will properly nose over when it begins its descending branch, again, this coming mostly from LR shooters. I had this conversation with the late Bob McCoy some years back, and he stated that essentially, no small arms projectiles really nose over, and that all remain with their axis remaining pretty much on the same line as the line of bore at firing. Artillery is really where the question becomes important, as they have a vested interest in making sure than nose-fused projectiles impact nose (fuse) first. Interesting topic, though!

 

[NHgunNut]

I tryed blowing up some 36 grain varmint grenade's in my .223 1-8" twist. the case's were completely full of h-380, right to the top and compressed the bullets in. they hit paper fine at 100 yards to my dissapointment. cases didnt like it too much however

 

[Randfal]

I had the exact same thing happen to me. Normally shoot the 105-108s and tried some 88gr Bergers in my .243AI 27" 1:8 3 groove Benchmark barrel. I couldn't push them faster than 3300 fps without them coming apart 30-40 yards from barrel. Neat little puffs and nothing on the paper. Slowed them down below 3200 and they would stay together. Bore is very smooth and nice throat transition. I don't think it was a rough barrel problem. I believe it is a combination of thin jackets and high RPM. As a side note, that same barrel will stabilize 115 Bergers enough for 3" groups at 600yds.

 

[flatlander]

My 1st experience with fast-twist & thin jackets came with a Colt AR15 (1:7tw) & Hornady SX 55s. You just know something's going wrong when you can't hit a 2' square target at 100yds with a previously zero'd rifle. Also tried some bulk WW64PSPs through the same barrel, and got impacts all over the berm - as far as 15' away from the point of aim, again at 100yds. This bbl. shot fine with every other bullet I used in it, so I never tried any of the thin jacketed varmint bullets in it again.

Most recent experience was with some older Berger 108s out of a 6x47 Lapua, which has a 30" Krieger 1:7.5tw, .236" bore. We were just finishing up a BR Varmint Silhouette match, which had been run with an uneven number of relays. I'd shot the crows at 300M, and since no one had shot at the ground squirrels at 385M, just moved right on over to them, without giving the barrel its customary cooling-off period while we re-set targets. Had some problems getting centered-up on the sighter target, as the rifle wasn't grouping as it usually does. When I moved on to the record targets, I hit the first couple, then had the next shot hit the rail, followed by a miss which didn't hit the berm (this bbl. has a Vias brake, so I can usually see the impact). Had a friend come over with his spotting scope, and he witnessed the next shot 'poofing' about 100yds. out.

I've since switched to S107MKs & DTAC 115s in this bbl, and haven't had any problems since. Borescoping shows that it's probably getting close to the end of its useful life, but as long as it shoots well with the 107s & 115s, I'll hang on to it. There's a 30" Bartlein 5R 1:7.8tw, .237" bore hvy. Palma blank on the shelf, ready to be fitted & chambered when this barrel's toast.

 

[Jim See]

2 points,

I have seen the puffs of smoke from f-class guns, about 75 yards down range a very distint
ring of smoke yet the bullet still impacts the 1000 yard target???? what is happening?

I blew up some hornady 70 grain SXHP in my 12 twist 243 AI. pretty new tube but velocity was exceeding 3700.

 

[glo]

Prompted by another thread Bullet 'attitude'
http://forum.accurateshooter.com/index.php/topic,3776096.0.html

I ran some numbers. Here is why the overstabaliztion of a bullet from a small arm doesn't happen.

Correct you are NHgunNut!!! 60 minuets equals 1 degree. In shooter speak, that means 60 MOA equals 1 degree. So a 20 MOA base plus the 10 to 15 MOA you add will get you out to about 1000 yards.

So, your avarage scope has about a maximum of 15 MOA of elevation or about a quarter of a degree of elevation.

So, your avarage target bullet being between 1.2 and 1.4 inch long will be exiting the muzzel at a half degree (for a 1000 yard shoot), will only be titlid up off its axis about .0108". In other words the holes made by bullets at 1000 yards are going to be .0054" out of round or look round to the naked eye.

Slight correction: the bullet will be tilted up off the line of sight axis .0108"

 

[bozo699]

Prompted by another thread Bullet 'attitude'
http://forum.accurateshooter.com/index.php/topic,3776096.0.html

I ran some numbers. Here is why the overstabaliztion of a bullet from a small arm doesn't happen.

Correct you are NHgunNut!!! 60 minuets equals 1 degree. In shooter speak, that means 60 MOA equals 1 degree. So a 20 MOA base plus the 10 to 15 MOA you add will get you out to about 1000 yards.

So, your avarage scope has about a maximum of 15 MOA of elevation or about a quarter of a degree of elevation.

So, your avarage target bullet being between 1.2 and 1.4 inch long will be exiting the muzzel at a half degree (for a 1000 yard shoot), will only be titlid up off its axis about .0108". In other words the holes made by bullets at 1000 yards are going to be .0054" out of round or look round to the naked eye.

Slight correction: the bullet will be tilted up off the line of sight axis .0108"

I must have above average scopes because all my !k yard rifles have 20 MOA bases on them and with a 6*284 I have to add 21-22 MOA to get to 1K yds my slower 6brx I think is about 24-25 MOA and my .308 needs about 37 MOA I have no problems getting there. I think Kevin Thomas is on the right track.
Wayne.

 

[expiper]

Just because a bullet is started "uphill" at a certain angle or MOA....doesnt mean that it drops on line of trajectory at the same ammount......they are commin down like a rock wayyy out there...Roger

 

[bozo699]

Just because a bullet is started "uphill" at a certain angle or MOA....doesnt mean that it drops on line of trajectory at the same ammount......they are commin down like a rock wayyy out there...Roger

Absolutely Roger
Wayne.