Thread: Barrel-bore enlargement

I have a Sako 75 .308 std contour barrel and a Sako 85 7mm Rem Mag, std barrel also.
I want to use an ASE SL7 .30 suppressor for both rifles. I'm going to cut 2in off both rifles and get the same thread cut. I'm worried about the bore enlarging on the .308 barrel after threading due to not much barrel to cut into.(its about 16 mm).

So my questions are; What threads have people got cut on their Sako barrels? Has anyone experienced the barrel opening up on the end where the thread is cut? And do the ASE suppressors butt up against a shoulder cut into barrel or up against muzzle?

I read on Dean Maiseys website its common on buttoned-barrels to open up at the end.

No haven't got suppressor yet, waiting to speak to gunsmith about thread size on particular rifle. Was hoping someone on here might be able to share personal experiences regarding my queries first. I'm going to cut 2in off, I've shot enough to figure out that 100fps aint going to make a difference in the real world at the distances I shoot.

What do you mean by the barrel "opening up?

Originally Posted by Toby

What do you mean by the barrel "opening up?

Some barrels will open up at the end of the barrel where the thread is cut due to bigger bore size eg 30cal and to much steel taken off from the outside due to thread. On smaller contoured barrels.

This is copied from D Maiseys website;
All DFM muzzlebrakes feature an internal shoulder, so muzzlebrake can be seated either in the conventional manner to pull up tight on the 'shoulder', or on the muzzle/nose (actually the best method). This permits the gunsmith to use the maximum thread size possible for the diameter of barrel at the muzzle. This provides better strength, less chance of barrel stretching (due to unnecessary over-tightening by rough operators), and less chance of the barrel-bore enlarging under the threaded section at the muzzle (common problem on button-rifled barrels).

There is another article on accurateshooter.com if you feel the need to know more.

This one :
Muzzle Threading — Don’t Remove Too Much Steel « Daily Bulletin

First , sako barrels are hammer forged not button rifled.
Measure the outside diameter of your barrel where the shoulder will end up( typically 12 to 15 mm from muzzle) and you can then comfortably have 1 to 1.5 mm of metal removed from that measured diameter with out worrying to much about the internal dimensions of your bore.

You will notice on the accurate web site article that it concerns noticeably fat button rifled barrels where a good amount of meat had been removed from muzzle.

Actually the sako/ tikka stainless barrels are pretty easy to turn down and get a nice finish thread onto.
The old hammer forged chrome moly BSA / parler hale and old Remingtons can be a bit more rough sometime.

I supposed that the metallurgy and heat treatment have evolved since the 60's and the steels are easier to machine and have a better finish and the cutting tools doing the work are better as well.

Robbie Tiffen at Gunworks offers his suppressors in 9/16" UNF (18 TPI) which is about 14mm leaving plenty of meat at the muzzle. His suppressors also have a rebated thread meaning that a runout groove is not needed to get the correct shoulder seating. Having said that, given that your rifle is a .308, and 1/2"x 20 muzzle thread is not going to be a problem as the muzzle pressures are quite low on a standard length barrel.

Originally Posted by gundoc

Robbie Tiffen at Gunworks offers his suppressors in 9/16" UNF (18 TPI) which is about 14mm leaving plenty of meat at the muzzle. His suppressors also have a rebated thread meaning that a runout groove is not needed to get the correct shoulder seating. Having said that, given that your rifle is a .308, and 1/2"x 20 muzzle thread is not going to be a problem as the muzzle pressures are quite low on a standard length barrel.

What about the 7mm maganum? I was wanting to put the same thread on that as well.

Go with 14x1 if 9/16 x18 or x24 is not available . 9/16 is 14.29mm.
14x1 is one of the standard threads of ase ultra. There should be enough meat on both barrels to do that thread from memory.

http://www.aseutra.fi/assets/files/p...n-mitoitus.pdf
Good luck.

@gundoc you talk about muzzle pressures being lower in a std barrel 308. Being a 7mm mag are the pressures great enough to cause a problem?

Originally Posted by Friwi

Go with 14x1 if 9/16 x18 or x24 is not available . 9/16 is 14.29mm.
14x1 is one of the standard threads of ase ultra. There should be enough meat on both barrels to do that thread from memory.

http://www.aseutra.fi/assets/files/p...n-mitoitus.pdf
Good luck.

Yea Ive read that cheers, was hoping there would be enough for a 14x1, Im nervous about hacking into 2 expensive good shooting guns for it to go wrong accuracy wise. Ive had another rifle in 7mm mag with a button barrel threaded a few years ago that didnt shoot anywhere near as good as it did before threading.

Originally Posted by Frosty

@gundoc you talk about muzzle pressures being lower in a std barrel 308. Being a 7mm mag are the pressures great enough to cause a problem?

The muzzle pressure is higher in the magnums than the .308 but not really enough to cause muzzle expansion (unless there is some foreign body in the muzzle such as dust, dirt, water, etc). Muzzle expansion is usually caused by a combination of tough steel that has been compressed a little by button rifling (this technique produces excellent barrels but involves displacing the bore to form lands and grooves from a hole that is roughly half the distance between the land and grooves, ie: a .308 has a nominal .300" bore and .308" grooves formed from a hole of about .304"). The reduction of the muzzle to 1/2" for threading can release internal stresses which can often allow a diameter 'creep' of a small amount. This is usually not enough to cause any significant drop in hunting accuracy but could be noticeable in a target rifle. Stainless barrels are considerably softer than chrome-moly barrels (which is why s/s barrels don't last as long as c/m) and the phenomenon is less obvious. The 9/16" threads are superior in this regard, particularly on c/m barrels.