105's in a 10 1/4 twist?

[BADUNAME63]

I shoot an old win converted to .243 and the barrel it has now I "rod checked" at about a 1 in 10 1/4 twist.. It shoots 100 btsp's and 95 sst's just dandy. I recently tried the 105 A-max's and they all keyholed. Barely on paper. Then I just randomly thought of pulling the tips out and magically they stabilized just fine. Shot about 3/4 moa. And more recently I started cutting just about a 1/16th of an inch off the tips and its shooting 1/2 moa, no bs... Just wondering if anyone can explain this to me or if its some kinda magic....? powder and primer loads were the same for the keyholer's and the great shooters...

 

[Chief 101]

It is magic...its all about the length/diameter/weight. There are many books on exterior ballistics out there smarter than am I so I will leave the details to them. There are magical mathematical formulas to explain all this stuff. I sounds like you need to select a little different bullet...there are some heavier 6mm bullets out there that will work in a 1/10"twist, stock from the factory.

 

[BADUNAME63]

Well, I appreciate the response and I'll look for another bullet in the 105 range. Ballistically, I like the ability to reach out to 1000+. The A-max is available at my local store and its the biggest 6mm they have in stock is all.. I guess I'll try grafs... But i still wonder why just flattening the tip a little bit makes them stabilize? I didn't change seating depth and they are still just up against the lands at the top of the ogive.. it seems like the resistance caused by the blunter nose is giving it stability.... Does that make sense? I dont know but i know it works...

 

[ironworkerwill]

shorter bullets are easier to stablize regardless of the shape of the tip. lighter bullets stablize easier than heavy bullets. a faster bullet will stablize better than a slower bullet of the same length and weight. I suppose if you could push the 105 grn bullet out faster (if staying within safe loading guidelines are possible) it would stablize the bullet and not keyhole.

 

[BADUNAME63]

What im running is 40 grains of RL-19 in RP brass and fed mag primers... So, its more likely the 16th of an inch off the bullet length is causing the added stability? more than some added resistance from the blunter nose?

 

[dmazur]

The Greenhill Formula

http://en.wikipedia.org/wiki/Greenhill_formula

shows that a faster twist rate (expressed in inches per turn) will be required for longer bullets (because L is in the denominator.)

So, I believe you are correct in theorizing that the increased stability is due to shorter bullet length.

 

[BADUNAME63]

Okay... Thanks for that,.. pritty much cleared it right up for me. I went and did some calculations using that formula and it can work in a 10 twist if you can get it up to the 2750 to 2800 fps mark. which is what im figuring my MV is. Maybe as low as 2700 fps. Thank's all for the replies, and for those who didn't know, if you want to gain the ballistic advantage of a 105g A-Max through a 10 twist barrel, simply snip all but a sliver of the red tip off and then smooth it over to form what looks like a soft point made of red lead.... And Whalla (?) you have a functional 105g bullet for long range shots.......

 

[gamestalker]

I was just reading up on some of that in preparation to load some 6mm Rem. , and what I understand is when the twist gets above like 1 -9 1/2 you have difficulty stabilizing those longer bullets. Back when the 6mm was introduced it often had a 1-12 twist and would stabilize up to 80 gr. bullets very well, but not the longer 90 gr to 105 gr projectiles. So I'm thinking that barrel you have @ 1- 10 1/2 twist is too much for the 105's as well. You might be forced to stay away from those longer profiled 105's in that barrel.
GS

 

[BADUNAME63]

I would most certainly agree with you... A 10 twist is not supposed to stabilize a 105 typically, however, I will say with absolute certainty that tho cutting down the tips may take away a small amount of the B.C., its not so much that they are not still great long range bullets. I have made this modification in order to shoot these at long ranges since they still perform better than the 90g fmj-bt at those ranges... So in essence, if you have a 10 twist and want the ballistic advantage of the A-Max, cut off most of the tip and then smooth it over, then you should be able to shoot them... It works for me at least...