Flat based bullets and muzzle crown

[cebuboy]

Hello guys, not sure if this is the right section but it does involve reloads though…

Do flat based bullets prefer a particular muzzle crown?

I’m currently racking my brains for an explanation as to why my loads work on some barrels, works most of the time with a group of barrels and plainly just does not work with a particular brand of barrels.

I’m using a locally made flat based bullet, weighs 55 grains, length is .708"/18mm, diameter is .223/5.68mm.

pics of the bullet, pardon the crappy cellphone pics…

Powder used is 24 grains of VV N133, russo primers and assorted brass.

From various chrome lined BCM barrels, 2 11.5", 1 14.5", 1 16", good groups at 100m and 170m were achieved and sub moa at 100m from an SS410.

An LMT 14.5"chrome lined also did good at 100m and 170m.

A Bushmaster 14.5" 1/7 M4 style does group ok at those distances most of the time, an average of 2 key-holing fliers for every 10 rounds… So does a Noveske N4 light…

It just does not work with Daniel Defense(pre chf) barrels, tested are 1 16" mid-length, 2 16" M4 cut with carbine gas… Hits at 30m are keyholes if ever they get to land on a classic IPSC target board, never mind at 100m. The rounds I think are not stabilized as one barrel’s muzzle brake got a hit on the exit baffle. However, with one 16" M4 cut that was cut down to 14.5", the groupings are good.

All barrels shoot fine with boat-tail bullets, 55 grain fmj-bt, 77 grain smk, 75 grain A-max and also M193 and M855…

So this got me thinking if the crown design has anything to do with this…

Any inputs are highly appreciated…

[Clint]

How does the bore dia at the muzzle look?

Try dropping a .219" pin gauge into the muzzles.

[cebuboy]

@Clint

How does the bore dia at the muzzle look?

Try dropping a .219" pin gauge into the muzzles.

Have not measured the bore diameters at the muzzle yet, as they shoot fine with boat-tail bullets.

[AJS]

nothing to do with crown type. No one has been able to show one style is better than another for anything IF they are cut correctly.
Have you checked the actual twist rates?

[cebuboy]

All the barrels have 1/7 twist.

Tried this bullet on a cut-down Elisco A1 1/12 twist barrel, very tight groups at 30 meters.

I’m wondering what could be the cause of this, as those barrels shoot well with boat-tail bullets.

[markm]

Why are you burning fancy powder and racking your brains trying to get some “locally made” bullets to work good?

Shit! If I’m going to pay for that high falootin powder, I’d be running those A Max or V max bullets that are accurate in that bullet weight range.

[cebuboy]

For now, that is the only brand of rifle powder available to us and that bullet is the cheapest available here for us to shoot on a regular basis. Sure them A-max and V-max bullets are nice but it is very costly to shoot regularly for practice…

I am thinking what else is there that we have overlooked. Why it works well on some and not on others. Aside from the lone 1/12 twist barrel, the rest were all 1/7…

[tomme_boy]

Normally, the FB bullets will handle a crown that is damaged better than the bullets with a BT. The FB bullets will release faster than the BT bullets. The theory is the BT is still in the bore and can be influenced by escaping gasses. With the FB it is not influenced as long so the bullet is more stable.

I would have to think that the bullets are to blame. Check to see what the diameter is on the bullets. Also, they could have a loose core. This would cause the unstable condition you say. The jacket could be thin and is tearing the bullets jacket up. Seen this happen in 3 groove barrels and shooting thin varmit bullets.

I would just buy some better bullets.

[MistWolf]

Every barrel is a law unto itself, even when testing barrels of the same manufacture. However, it’s a bit strange that the bullets are keyholing. Easiest thing to do is test the barrels with another bullet. if the problem goes away, it’s likely to be that the bullet producing keyholes is incompatible with that particular barrel. There are avenues to pursue to see what’s causing the problems, but if the barrel only keyholes with the one bullet, it’s simplest to switch to another bullet

[cebuboy]

@MistWolf

Every barrel is a law unto itself, even when testing barrels of the same manufacture. However, it’s a bit strange that the bullets are keyholing. Easiest thing to do is test the barrels with another bullet. if the problem goes away, it’s likely to be that the bullet producing keyholes is incompatible with that particular barrel. There are avenues to pursue to see what’s causing the problems, but if the barrel only keyholes with the one bullet, it’s simplest to switch to another bullet

MistWolf, All barrels shoot fine with factory ammo and with the mentioned Sierra and Hornady bullets. What I am trying to figure out is why the BCM and LMT barrels shoot them fine, there must be something about them that make them behave that way. With regards to the DD, all three of them just don’t like that bullet. And another puzzling piece is the Bushy and Noveske N4 light, they can shoot it well most(80%) of the time. The only thing I can think of is the crown… Well, this is worth studying some more…

@tomme_boy

Normally, the FB bullets will handle a crown that is damaged better than the bullets with a BT. The FB bullets will release faster than the BT bullets. The theory is the BT is still in the bore and can be influenced by escaping gasses. With the FB it is not influenced as long so the bullet is more stable.

I would have to think that the bullets are to blame. Check to see what the diameter is on the bullets. Also, they could have a loose core. This would cause the unstable condition you say. The jacket could be thin and is tearing the bullets jacket up. Seen this happen in 3 groove barrels and shooting thin varmit bullets.

I would just buy some better bullets.

tomme boy, So in theory FB bullets are better than the boat tails, it did well on a cut-down barrel with a 90 degree crown…

Yes, I know that it is the bullet that is suspect here, I’m just trying to narrow down the variables. Bullets measure .223"/5.68mm. The loose core is worth checking out, but then again the BCM barrels shoot them just fine all the time… Will check out their respective crowns when I have the time. I can understand if the variance is from barrel to barrel, but it is brand to brand. So there must be something BCM and LMT are doing why their barrels can shoot it.

Thanks for the responses.

[AJS]

@cebuboy

All the barrels have 1/7 twist.

Tried this bullet on a cut-down Elisco A1 1/12 twist barrel, very tight groups at 30 meters.

I’m wondering what could be the cause of this, as those barrels shoot well with boat-tail bullets.

It’s really, really simple. One of your barrels can’t cope with those bullets. One of my 1 in 7’s will not shoot anything that light.
It would also be interesting for you to CHECK your twist. Barrels are often not what they are claimed. It should only be a little bit out but you never know. Crown has nothing to do with this.

[MistWolf]

Now that I think of it, AJS is right. If the crown were throwing the bullets off that much, other bullets would be affected as well.

I’m thinking maybe the jackets are soft & don’t like the rifling. Bullets could be skidding

[cebuboy]

@AJS

It’s really, really simple. One of your barrels can’t cope with those bullets. One of my 1 in 7’s will not shoot anything that light.
It would also be interesting for you to CHECK your twist. Barrels are often not what they are claimed. It should only be a little bit out but you never know. Crown has nothing to do with this.

AJS, I can understand if it is one of them, but all three(1 16" mid-length and 2 16" M4 profile) of them from the same maker, makes me wonder why. And we have one 16" M4 profile that was cut-down to 14.5" that shoots fine…

@MistWolf

Now that I think of it, AJS is right. If the crown were throwing the bullets off that much, other bullets would be affected as well.

I’m thinking maybe the jackets are soft & don’t like the rifling. Bullets could be skidding

Was thinking about that also… But, in all the BCM(4) and a lone LMT, the bullet groups fine…

[AJS]

Why? Because some barrels do not like some bullets.
If it shoots other projectiles accurately forget about it and move on.
If the crown was off ALL the projectiles would have issues.

To check the twist, put a rod with jag/patch or brush in the bore some distance. Mark a position on it with a pen, draw it out slowely so it does not skip and mark when it’s done one complete turn. The distance between those points is your twist. Do it for all of them and you will know the actual twist. If it’s not way out then it’s time to move on This is how things work, sometimes you have to change projectiles.

There really is nothing you can do to solve this. It is how it is.

[Wolverine954]

Try seating a few real long and running them thru the problematic rifles. Go as long as you can, single loading. If they group well, then it isn’t a crown issue.

[shootist1]

Barrels are always a crap shoot. Nothing unusual about one not liking a load that others shoot well and vice versa. Sometimes you get lucky and sometimes you don’t.

Harmonics, chamber differences, CL coating, reamer wear, luck of the draw, who knows?

[cebuboy]

Update:

Had the 16" mid-length barrel re-crowned with a 90° target crown, seems to shoot fine at 30 meters with an A2 fh…

Also, we tried one of the 16" M4 style barrel with the muzzle device(phantom brake) off, the groupings improved and the keyholes disappeared.

Now, will see how they do at 100m…

Will take pics of the muzzle crowns later.