Grouping Issue

[LukeSoutherner]

Right, I've finally gotten a new moderator (Tier One: Spartan 3) for my DTA SRS Covert .308 (and my other rifles: see other post about moderators for duel calibers!). This is an accurate rifle (i.e. groupings of at least less than one inch, to keep it simple, but really 0.5").

 

Today, shooting without the moderator or muzzle break:

 

4 shot group = 0.632"

 

I then put the moderator on, and I got this:

 

4 shot group = at least 4.5"

 

Throughout my load testing today the rifle was struggling to group with the moderator on, but was fine it with off.

 

Any ideas as to why this is happening would be much appreciated - I'm stumped!

[1967spud]

IS IT HITTING THE MOD?

[LukeSoutherner]

No Mark, it ain't...thankfully!

 

Last moderator did not fare so well.

 

More ideas please!

 

The barrel has a 1:8 twist, which is pretty fast for a .308. I guess it was designed for subsonics - not useful for deer! I wonder if this is an issue. The primers are not flattened, but do show cratering, even with a light load.

[TattooedGun]

What do you mean, your last mod didn't fare so well?

If your last mod got hit, there's a fair chance the thread is not cut concentric to the bore, you could be lucky and the tier one exit hole is a gnats whisker wider than the last mod, so it's just clipped by the bullet < 0.001" - enough to open the group considerably, but not enough for total failure.

[LukeSoutherner]

Ah, you could have a point. The last mod did have baffle strikes, but I thought this happened because the mod loosened from the muzzle whist firing (I thought I was a bit liberal with copper lube on the thread).

 

I'll be disappointed if the thread hasn't been cut concentrically considering this is a factory rifle.

 

I should mention that the mod works perfectly on my 6mmBR.

[justin credible]

 

I should mention that the mod works perfectly on my 6mmBR.

 

That's because a 6mm isn't as fat as a .308!

[gun nut]

That's because the thread is cut right on your 6BR, and obviously not on your .308!!!!!!

[LukeSoutherner]

Thanks for your input guys.

 

How do I go about checking if the thread is concentric to the bore then? I'm guessing this is a gunsmith's job...

[mikeroz]

Bit of a trek but about 100 miles from you is Mik Macsimovic of Dolphin Guns. His credentials speak for themselves. He will be able to establish once & for all if it's a theading issue on your 308.

No b*ll sh*t from Mik. Straight talking honest expert is he!

 

Or, if you're after closer Valkyrie Rifles is 53 miles from Halifax by Google maps.

[Swarovski1]

I too would say bullet is clipping the mod exiting.i had this with a muzzle brake.the hole made a few thow bigger and bingo shooting strait again.

[LukeSoutherner]

Thanks Swaro. I wonder if the hole in the moderator's baffles could be machined a bit bigger to sort that out, as re-threading will cost more I'll lose some length off the barrel too.

[TattooedGun]

Not being funny, but I'd imagine you'll spend far more trying to get the individual baffles machined on the moderator than getting the thread aligned to the bore.

Yes, you might lose 1/2-3/4" off the length of the barrel, but you'll know it's straight and have no future problems with any mods.

 

The way that I tested when I had the same issue was to put a bore guide in the action and with the mod on I put a cleaning rod through the barrel, if it doesn't come out in the centre of the exit hole on the moderator, then it isn't threaded straight (provided your rod is straight...!).

 

Cant vouch for the guys close to you, but Amanda of Wildcat Moderators (UK Custom Shop) got my rifle re-threaded by their gunsmith for not a lot of money at all even though the fault was with my previously cut rifle, and not the mod they provided.

[hunter686]

It sounds like the threads are off on your rifle.

 

When i was making my mod i used to check my concentricity by doing the following;

 

Take a cleaning rod and push it all the way through the bore. Wrap two bushes about 2 inches apart (leave a mod length plus an inch or so at the end. Pull the snug bushes into the bore until they are both engaged.

 

2) Screw the mod on! Once the end of the rod has come though wrap another bush around the end about half a mm smaller than the caliber.

 

3) Pull the half mm smaller bush back into exit hole of the mod. If the hole isn't bang on with the exit hole of the mod. Your threads are jacked!

[activeviii]

it might not be the thread, it can be the shoulder that is not true. it the thread is off but the shoulder it true then as you tighten the mod to the shoulder it will pull it square, that is if there is a little slope in the thread.

one more thing it might be is the barrel harmonics have changed with a change of mod

sort the barrel out first, if the Tier One: Spartan 3 is strippable then have very close look at the holes in baffles and the very end. i have painted the edges with white marker pen before now and put a single shot down, then had a closer look to see the paint was rubbed off on edge. it was the shoulder on the rifle in my case. gave up and just had chris - screw a new barrel on.

[baldie]

A bullet doesn't have to clip anything to ruin the group. If the mod is not square to the bore, uneven gas pressure inside the mod can throw the group....it doesn't have to touch.

 

The barrel needs to go in a lathe, have the bore clocked in, then the shoulder checked for square by screwing on a ground ring gauge, and then clocking that.