6br accuracy loss from 150 to 200

[Longrange57]

OP has the answer. Have not shot a Speer in many years. However, a Speer is not a Sierra or a Berger. If the Sierra is shooting good at 200, then don't shoot the Speer at that distance. Dusty is probably right, something is up with the bullet!

 

[Texas10]

I shot both the 70 grainers on 2 different days just to check. Speer and Sierra both shoot great @ 150 and Sierra shoots great @ 200 but TNT sucks. Conditions same both days. If the TNT was coming apart, would it not come apart in the first 150 when vel. is highest?
Anyway, this load is lost to mysteries of the universe.

The 306,000 rpm rotational speed does not decay in proportion to velocity, so the TNT's thin jacket could well be starting to come apart between 150 and 200 as the bullet loses velocity but the RPM barely changes. The fact that other similar bullets have no problems suggest this particular bullet made with a thin jacket in order to explode varmints into 'pink mist' may not be able to handle that rpm quite as well as others. A close inspection of target paper may reveal some telltale evidence, particularly if a 300 yd range is available.

 

[Lefty Trigger]

Speer also puts limitations on some of their bullets pertaining to speed, have you checked that? I'm not sure about the twist rate being wrong except with your bullet choice. I have shot Sierra 60gr bullets out of a 30 inch long 8 twist and they clock over 3600 fps and shoot pretty damn good out to 300 yards, ideal,.. no but it does work well. May be the extra 50 yards was all they needed to become unstable? Try reducing your load some and try a few going slower and see what happens, crank a few down to say 3000 fps and see how they do.

 

[SSE]

Thanks Texas10 and Lefty Trigger,
I will try loading some down just to see what happens, next time there is virtually "no wind"

 

[gunsandgunsmithing]

There is always wind. I love when people say they shoot when there is no wind. The only way there is no wind is if you shoot in a tunnel or a chicken house. While your shooting those bullets that your shooting they are not the optimum bullet weight for the twist that your using. Also what Dusty said. Get some flags.

Yep, without flags, you can't know what the wind is doing. So, saying there was no wind without flags, coupled with no other logical explanation for groups opening up during the last 50 yards, well...Shooting without flags is a waste of powder, bullets and barrel life. You can't learn from what can't been seen.

 

[SSE]

I am pretty lucky in that my range is in a valley with trees on both sides and tall grass that act as my flags.
Just got back from PD shooting in Wyoming and sure could have used some flags out there! Sometimes blowing in 3 different directions at once.

 

[Lefty Trigger]

That is the problem we have at our range as well, the wind is all over the place. At the bench it's at your back, 100 yards down its at a 45 blowing NE and at 200 yards down its mostly east and at the targets is blowing at you or NW. We put flags up once and it was a pure nightmare trying to figure them all out and which ones would have the most affect on the bullet path,... we all gave up.

 

[pdhntr]

I could swear 30 years ago, they had a "speed limit" on the 70 TNT box. Don't have any left from that time period, and my memory, well..........what the hell was I talking about?

Regardless of the issue, I can assure you that loaded at a reasonable speed, the Speer 70 grain TNT is arguably the best 6mm coyote bullet ever made. I killed a pile of coyotes with them from a Featherweight 243 over a 20 year period and it was the least sewing I have ever done. I have never had a bullet perform more consistently.

Jim

 

[Evan]

No wind. I am shooting off my back deck with targets no more than 10 degree change of angle between 150 and 200. The 70 gr. Sierra is shooting less than .5 moa when I do my part @ 200 with same load. Both the Sierra and Speer are shooting WAY under .5 moa at 150. I have repeated this several times on 2 days.

It takes a mighty wind to blow bullets out a couple inches between 150 and 200. I'm going to double down on my assertion that your bullets are coming apart from excessive rpm. Why not pick up a box of 90-105gn bullets and give them a try? They'd be much better suited to your twist rate.

 

[LHSmith]

when your bullets have stability issues, then wind effects become even more pronounced. FWIW the best groups come from shooting in the same condition. Calm is not the optimum, as there is always something going on between you and the target, many times you just can't see it. With a consistent wind condition most likely mirage will also be close to the same from shot to shot. Don't discount mirage effects as it can change at various distances. Use wind flags and learn how to read them. Without them, you are just guessing whats happening out there.

 

[Ledd Slinger]

Yeah I’ve tried various 70gr and 75gr bullets in a 1 in 8” twist 6mm Creedmoor and 6mm Remington. They don’t work well with that twist.

Just picked up some Sierra 85gr Varminter bullets and expect they will work better

 

[VA_XTC_Shooter]

If it is really, honestly shooting 0.5" at 150, then I'd expect no more than 0.75" at 200. Going up to 2" in the last 50yds sounds like the bullets are failing.

3400fps in an 8 twist is 306krpm. Could the bullets be coming apart? Maybe not entirely going poof, but jacket and core are starting to separate and destabilize? Many bullets can go poof at 300krpm and above.

I am betting on bullet failing. I have not Speer TNT in the 6BR, but I did "try" to shoot them in the 223 AR. No bueno. No matter how light I loaded them, in a 1:7 twist barrel, about 1 in 4 would vaporize leaving a gray contrail to the 100 yd target. However, the ones that made it to target were accurate.

look at the size of the open tip on those TNTs, no wonder they are sensitive to twist and velocity and all that centrifugal force put on them.