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Barrel does not like moderators?


walkabout

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I have been trying to get my new 7mm SAUM to group better than .75" at 100 yds during load development.

During the process I have been using a Moderator, If i take the Mod off the groups improve by about .25".

I tried a different Mod, the same thing happened, group opened up with mod, shrank with it off.

The barrel is a Benchmark 28" in a Medium Palma profile.

I've never had this happen before, in the past groups have either stayed the same or improved a bit with a mod.

Anybody had a similar experience or have an idea why this would be happening?

I thought maybe the Mods are too heavy for the profile?

Mark

 

 

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which moderators?

 

Sounds like either wonky mods (unlikely with two separate units)

 

Or perhaps muzzle thread slightly off.

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How many groups did you fire with and without the can to come to the conclusion that without it groups are 1/4" bigger?

 

I would think that if the threads are wonky, then the groups would be considerably more than 1/4" bigger

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You need to alter your cartridge OAL when you fit a mod. The extra weight frigs about with the harmonics, the last time I changed the mod on my 20tac I went from an extremely small light mod to a not so small extremely heavy mod (needed better sound moderation) and the oal on my load had to be adjusted 15 thou to shrink the groups to what they were before. In this case 15thou longer OAL but it will vary from rifle to rifle.

 

Mike

 

PS if you're tight to the lands you may need to adjust powder load +/_ .2grains

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Thanks for the replys

The same maker of mods, one heavy one light.

Have checked the thread and it seems okay, I have screwed on two other mods with the same thread (different make and calibre) and they seem fine as they go on.

I was firing lots of 5 shot groups.

Barrel is free floating in a XLR stock.

I might try a different maker of mod and see what happens?

Of course it could well be the "Nut behind the butt"

 

Mark

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The only way to check the thread Mark, is to mount the barrel in a lathe,indicate the bore in, and clock the shoulder face. Simply screwing different mods on , won't tell you anything.

 

The shoulder is the critical part of a muzzle thread, not the threads themselves, though, if cut to a ring gauge, should give the necessary clearance.

 

People think tight, is good. If a thread is tight inside a mod, its out of spec. Mods are cut generously.

 

looking at your group sizes again, I would think the problem lies elsewhere. 1/4" doesn't mean there is anything mechanically wrong.

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Are you saying that during load development the groups are generally 1/4" bigger across all powder charges and seating depths?

 

Ive seen accuracy wander when changing between two mods of considerable weight difference and put this down to it changing the harmonics, as gun nut pointed out I needed to change my load to suit the new mod.

 

As Dave has already said, its the shoulder than needs to be square to the bore, within reason the threads are just there to make the connection. If the shoulder is out of square then it will be out of square for every moderator you screw on and the only way to find this out is clock the shoulder up properly in a lathe with the bore running true.

 

Is there a pattern to the groups with and without the mods? when the mod is on do they both throw the groups out in a similar way or is everything just random?

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If the mod was being touched by the bullet, there would be obvious witness marks and wild inaccurate patterning to the "group"

 

1/4" deviation away from a "regular" group with or without mod is probably down to the barrel harmonic changing.

 

I'd suggest going back to seat depth / powder charge / bullet / primer and start again.

 

Its quite possible that the barrelk in question doe not like what your feeding it in current form.

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If the mod was being touched by the bullet, there would be obvious witness marks and wild inaccurate patterning to the "group"

 

1/4" deviation away from a "regular" group with or without mod is probably down to the barrel harmonic changing.

 

I'd suggest going back to seat depth / powder charge / bullet / primer and start again.

 

 

 

Its quite possible that the barrelk in question doe not like what your feeding it in current form.

I'd also suggest trying some other loads, .75 from a custom 7mm Saum isn't the norm in my experience of the calibre.

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Thanks guys

I think it could be the load.

I will think again, I have be loading to mag length of 2.930" in a Modified AI mag in a short action.

But I seem a long way off the lands, so will load longer and get closer to lands, I may have to hand feed it.

 

Mark

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