The rifle is an Interarms Mrk-10 chambered in 240 Gibbs. Barrel make unknown. Crown is good so far as I can see and is a standard shaped crown. This barrels twist preferres 100 gr 6mm bullets over all other weights. It shoots flat base bullets into just under MOA. But a boattail wont shoot, any weight any make dosn't matter.

Brass used is reformed LC Match and neck turned. Velocity in the 3350 range.

So here's my question: what properties in the barrel or elsewhere would cause a preference for one bullet rear end shape over another?

I've often seen flat base outshoot BT. My guess is the shape of the tail and the way gas passes but it as the bullet leaves the muzzle. The one time I couldn't get BT bullets to shoot in a rifle it was a crown issue. But don't take my one time as the rule.

As I understand it, boattails need a couple hundred yards to stabilize, flats don't; once stabilized, boattails come into their own producing greater accuracy at extended ranges.

If you're shooting at 100 yards, this might be an answer.

I've been wrong before

quote:

Originally posted by ramrod340:
I've often seen flat base outshoot BT. My guess is the shape of the tail and the way gas passes but it as the bullet leaves the muzzle. The one time I couldn't get BT bullets to shoot in a rifle it was a crown issue. But don't take my one time as the rule.

That has been my experience and take on it also. If the crown is not perfecty square or if the if the tail of the boat tail is not concentric or perfectly round, I think the gases can kick the ass end of the bullet up, down, left, or right as it exits the barrel.

Are you trying to shoot Berger VLDs?

Go to their website and find the latest info on seating depth.

This is critical.

Depending on the ogive, seating depth can be more forgiving for flat base bullets than boat tails.

flaco

flaco, I have not shot any VLD's but have shot Berger, Sierra, Speer, Nosler, and others and its alway a flat base that will get MOA or better. All the boattails will group 2 in or more.

My logic (altho maybe twisted) makes me think that an Off crown would effect a flat base more than a BT.

And as Tin Can suspected all test were at 100. But I am reluctant to assign all the error to the gyroscopic stabiility not being at full potential yet. The error is just too big. Must be another factor somewhere.

A lot of the shortrange[100-200 yds] records are with boat tail bullets.
Butch

I think it might be the center of gravity diff. between them.Good Luck

Hello,
Might want to give Sierra a call, free one at that, and believe you will find them most helpful in any questions you may have. All of the above answers are the ones I have heard for years, but Sierra or other bullet mfg.s may have a definitive answer.

I really don't know why some rifles shoot BT's well, and some don't. But, I think the centerfire benchrest guys are using flat based bullets today, but not 100% certain. I made my own 6mm bullets when shooting bencrest cometetively, and they were flat base bullets.
But, the Sierra 22 caliber 55 grain HPBT, their 90 grain 25 caliber HPBT and their 100 grain 7mm HPBT bullets all shoot better than any other bullets in my respective rifles in those calibers than any other bullet, except for Speer's bullets in the same weight range. Speer bullets shoot very well.
Don

Draw your own conclusions but it has been my experience that barrels of average to poor quality will group well with flat base bullets and not boat tail design.

Conversely high quality target grade barrels will shoot well with either one.

quote:

Originally posted by RayGunter:
The rifle is an Interarms Mrk-10 chambered in 240 Gibbs. Barrel make unknown. Crown is good so far as I can see and is a standard shaped crown. This barrels twist preferres 100 gr 6mm bullets over all other weights. It shoots flat base bullets into just under MOA. But a boattail wont shoot, any weight any make dosn't matter.

Brass used is reformed LC Match and neck turned. Velocity in the 3350 range.

So here's my question: what properties in the barrel or elsewhere would cause a preference for one bullet rear end shape over another?

IF the boatails are longer in the same weight, it could mean the twist is a bit too slow to completely stabilize a boattail configuration. This would be because the boattail bullet is longer. Try some shorter (lighter) boattail bullets. I once owned a 223 with 1/14" twist. It shot flat-based 55 grain softpoint bullets beautifully, but shot patterns using the GI ball 55-grain FMJBT bullet...the twist was just too slow for the longer bullet.

In addition, boatail bullets often have significantly less bearing surface in contact with the bore, so the barrel has less influence on the bullet as it travels up the bore. The boatatil design can become tipped in the rifling.

.

what tin can said...

The boat tail might not be concentric to the bullet.

quote:

IF the boatails are longer in the same weight, it could mean the twist is a bit too slow to completely stabilize a boattail configuration. This would be because the boattail bullet is longer. Try some shorter (lighter) boattail bullets. I once owned a 223 with 1/14" twist. It shot flat-based 55 grain softpoint bullets beautifully, but shot patterns using the GI ball 55-grain FMJBT bullet...the twist was just too slow for the longer bullet.
...

Amazing how many wrong answers were posted before the right one appeared. Since boattails are always longer than the same weight flat base design, they require more velocity or a faster twist in the rifling to be stabilized in flight.

quote:

Originally posted by The Shottist:

quote:

IF the boatails are longer in the same weight, it could mean the twist is a bit too slow to completely stabilize a boattail configuration. This would be because the boattail bullet is longer. Try some shorter (lighter) boattail bullets. I once owned a 223 with 1/14" twist. It shot flat-based 55 grain softpoint bullets beautifully, but shot patterns using the GI ball 55-grain FMJBT bullet...the twist was just too slow for the longer bullet.
...

Amazing how many wrong answers were posted before the right one appeared. Since boattails are always longer than the same weight flat base design, they require more velocity or a faster twist in the rifling to be stabilized in flight.

The length of the boat tail in regard to the twist rate is not the only issue. Most barrels are built with a plus tolerance so they are much more likely to be oversize than too tight.
The flat based bullet has a longer effective bearing surface and all other things being equal will be better aligned with the bore.

quote:

IF the boatails are longer in the same weight, it could mean the twist is a bit too slow to completely stabilize a boattail configuration. This would be because the boattail bullet is longer. Try some shorter (lighter) boattail bullets.

quote:

My logic (altho maybe twisted) makes me think that an Off crown would effect a flat base more than a BT.

Not twisted, no. At first glance one would think that but actually, a boat tail bullet has 'aerodynamic' characteristics that make it respond to the conical jet of gasses being blasted past it's base, more violently than would a flat base (providing that flat base is still flat and has no 'tails' from the rifling). So a crown problem or a bullet not riding completely true in the bore is going to do a wobbly! This is confirmed by statements like;

quote:

... it has been my experience that barrels of average to poor quality will group well with flat base bullets and not boat tail design.

quote:

In addition, boatail bullets often have significantly less bearing surface in contact with the bore, so the barrel has less influence on the bullet as it travels up the bore. The boatatil design can become tipped in the rifling.

quote:

Depending on the ogive, seating depth can be more forgiving for flat base bullets than boat tails.

quote:

... boattails need a couple hundred yards to stabilize, flats don't; once stabilized, boattails come into their own ...

quote:

The one time I couldn't get BT bullets to shoot in a rifle it was a crown issue.