No accuracy at faster speeds

[wv109323]

I have a CZ 75B Omega that I want to use in USPSA at the local club. The barrel is a 1:10 twist that has been throated and muzzle crowned. I test in a Ransom rest at 20 yards. USPSA rules require a 125 power factor to be within their rules. With a bullet weight of 125 g. that is 1000 fps.
The problem is that when I go above about 875 FPS the groups are terrible. I mean as bad as 14" with keyholing. This is consistent with 7 types of bullets( Lubed Cast, Powder Coated and Jacketed) and at least 6 different powders. I have sized cast and powder coated bullets from .355 to .3582. I have used Remington, Winchester and Hornady jacketed bullets (115 and 124g.)
Yesterday I tested my PC coated 120 Lee TC boolit with 6.5 g. of AA No. 7. These sized to .3570 and tapered crimp at .378. I always use Winchester Small pistol primers. I shot this load twice. Once at the beginning of the test session and at the last. Both groups were less than 2". I also shot the same boolit with 3.2 g. of Titegroup and it was less than 2 inches.
I then shot my cast and lubed 125 g. RNFP Lee mold boolit sized at .3582. I shot two groups one with 3.6 g. of Titegroup and another at 4.0 g. of Titegroup. Groups were 11 and 14" with about 50% of the boolits keyholing. This performance is typical with several bullets. Jacketed bullets will not keyhole but groups will be 4-6". Same with Blue Bullet PC and Bayou PC bullets at .355 and .356 diameters.
Can anybody make sense of this madness? Can a barrel act this way?
I have probably shot 70-80 test targets. I have not found a load of any bullet of any bullet weight that will group as the velocity approaches 1000 fps or greater.

[kungfustyle]

Did you slug your bbl? Know what grove diameter you have and go from there. What is the hardness of the bullet? Is your gun clean of copper, lead, carbon? All will effect your accuracy. Try cleaning it and shooting factory ammo out of it. If it still doesn't group call CZ and they may be able to help you out.

[gwpercle]

Try NOE # 358-124-TC-GC with gas check , sized .357 and lubed with Lithi-Bee bullet lube .
This is a 124 grain truncated cone much like the original design ... the gas check allows it to be driven to 1,000 + fps . The Lithium Beeswax lube and gas check prevent leading even with an alloy as soft as bhn 9 . I simply air cool them , size and lube .

My pet loads in 9mm luger with this boolit :
5.2 grs. Unique @ 1166 fps.
5.0 grs. W231 / HP38 @ 1135 fps.

I get 10 shot groups into 1 1/2 " at 50 yards with these loads if I do my part .
If these loads don't work ... I'm outta ideas , every 9mm luger I've tried these in has done well .
On a good day I've put 5 shots into a 3/4 " group . I really think the gas checked boolit helps with accuracy over 1,000 fps ... I had to nag Al Nelson at NOE for about 3 months to get him to make this one with a gas check ... but it solved all my accuracy problems !

NOE has ...In Stock ... a 2-cavity GC and a 4 cavity GC mould in this design if anyone's interested !
Gary

[Dusty Bannister]

With the barrel having been throated, are you certain that .357" is large enough? Another possible issue might be if the beginning of the lands have been moved deeper into the barrel by throating, the bullet might be unsupported as it enters the lands.

That is easy enough to find by the use of the old dowel rod and use the barrel as a gauge. If you are unfamiliar with that method, send PM and I will reply with details.

[wv109323]

Barrel slugs at .3562. Not a lot of jacketed bullets through the barrel and has been cleaned. No lead or copper The .3570 boolits work fine at the reduced velocities. The next sizer I have is .3582. In the last session the .3582 boolits were keyholing with 3.6 and 4.0 g. of titegroup.
Originally the rifling started at the end of the chamber. That is the reason for the throating so I could load a .3570 or larger boolit.
Gas checks may be an option.
Lead boolits are COWW with some tin added. I don't know hardness.

[Mal Paso]

I have a 1 in 10 barrel, use COWW with tin and tightened up my groups with water dropped boolets. I lost my hardness testing equipment but I used to get about 26 BHN with 44 cal. Drop the boolits directly from the mold into water and lube conventionally. If you PC they need to be heat treated again.

[joebaja]

Have you considered instead of going faster, going heavier? A 147 only has to be going 850 to make minor. I have had a few CZ clones - and other 9's with 1:10 twist. They all do better with heavier bullets.

[Willie T]

As gwpercle indicated with his loads, try a slower powder.
Willie

[PAndy]

I experiment with multiple diameters, multiple charge weights, multiple powder burn speeds. Hopefully results will tell the story.

[Cosmic_Charlie]

With the barrel having been throated, are you certain that .357" is large enough? Another possible issue might be if the beginning of the lands have been moved deeper into the barrel by throating, the bullet might be unsupported as it enters the lands.

That is easy enough to find by the use of the old dowel rod and use the barrel as a gauge. If you are unfamiliar with that method, send PM and I will reply with details.

This makes sense, see how far out you can seat your boolits. And don't over crimp your rounds, just enougn to chamber reliably.

[fredj338]

Change powders. I had this oddity with a BHP. It would shoot like crap with faster powders under lead bullets sized up or down. When I switched to Unique, groups were 2/3 smaller. A bad barrel or crown wont shoot anything well.

[whisler]

I had a similar experience with 9mm. Leading regardless of size with one powder (W 231 IIRC), but switching to Unique fixed it. Strange but true.

[Rapier]

Bar-Sto barrels for a match barrel fitted. What you have ain’t a match barrel.