How would you... ?

[memilanuk]

...extend the throat on an existing chambered barrel to a new specified dimension?

For example... I have a factory .308 Win barrel that is supposed to have an 'Obermeyer' chamber in it. I would like to have it throated a bit longer to better accommodate heavier/longer bullet (existing twist rate should be okay). Various prints from places like JGS, PTG, etc. show an 'Obermeyer' .308 chamber as having 0.085" freebore - the dimension from the end of the chamber to the start of the rifling. The people I've talked to who have guns specifically throated for say, a Berger 185gn Juggernaut, say they have '0.170" throat'.

That is what I want. How do I get there from where I'm at now?

Specifically... is it possible for a gunsmith to determine the throat length on an existing chamber? Unfortunately I don't yet have a dummy round from an existing chamber throated the way I would like, so I can't just supply the gunsmith with a dummy round and have the barrel chambered to match.

Down the road... I should have a dedicated reamer with the desired throat dimension, etc. but in the interim, I'm trying to figure out how to make the best use of what I have now, without the throat ending up some oddball dimension way off from what I want.

Thanks,

Monte

 

[Preacher]

You need to do a chamber cast with cerrosafe, you can obtain it from Brownells..
It will cast a 100% replica of your chamber, and into the rifling... and tell you or your smith everything you want to know..

 

[memilanuk]

Last time I had a cerrosafe chamber cast done it was pretty rough... I could have probably guessed more accurately than the dimensions that came off that thing. Maybe it was done incorrectly?

 

[TonyR]

Hi Monte.
My gunsmith has special reamers that just extend the throat. I have had no problem telling him "I need another 0.020 inches beyond my reamer freebore" and that's what I get. He did it with every new barrel when I was shooting VLDs out of my Shehane. A dummy round has also worked. It seems to be a real PITA to get a barrel that has already been chambered properly aligned to extend the throat because the range rods need to be longer but he has done that for me too along with a lot of words that don't belong on a family-oriented website. It is no problem when doing the initial chambering job on the barrel.

 

[gme]

With one of these
http://www.grizzly.com/products/T10471/images/
Gary Eliseo

 

[jscandale]

Monte,

If it's a .30 cal rifle and you can get the bbl off the action to ship it. I can give you the exact amount of throat that you want. It's really not that hard when you have the right tools for the job.

JS

 

[memilanuk]

Another 'fly in the ointment' is that I need to find out what size (diameter) the freebore is on the existing chamber... while most custom .308 reamers seem to spec a .3085" dia. freebore (or pretty close to it), the SAAMI spec is more like .310-.311". Not really enthused about the idea of a stepped/two-diameter freebore, though I'm not convinced it would be detrimental... just kinda hinky

Most likely the cerrosafe casting *would* be accurate enough to tell that from (I hope). If it does have a larger SAAMI freebore I think we'll just set things back 1/2" or so and start from scratch with a .308 Palma chamber reamer and a separate throater for the desired freebore. I may have some loaded round dimensions coming in from some folks with chambers similar to what I want, so if the existing chamber has a more reasonable freebore diameter we may be able to just get it dialed in and then throat it out a bit...

Thanks,

Monte

 

[Rustystud]

Monte:

I buy all my reamers with .20" or .40" free bore. My remedy to this freebore question. Is to use a MARS (Micrometer Adjustable Reamer Stop) and a piloted throating reamer. It can be done with the barrel removed and using a hand driver, or in a lathe. It also can be done with a PT&G Uni driver with a bushing in the action.
PT&G sells piloted throating reamers in 1.*, 1.5*,1.75*, 2.*, 2.5*, and 3.*. They also sell Uni Drivers and MARS.
Nat Lambeth

 

[Gundoktr]

The 308 Obermeyer reamer has a tighter freebore at .3085" and a standard length throat. If you know what bullet you want to shoot, just drop a few into a case, seated where you want and I can cut your freebore to accomodate that and give you any or no jump to the rifleing.

Jkob60@msn.com

Jim

 

[memilanuk]

Okay... time to back up a little and clarify something.

When I look at a reamer print, say for a .308 Obermeyer chamber, I see a section labeled 'lead' (I've also seen it labeled 'freebore') with a dimension like 0.085" between two points, and a section labeled 'throat' where it transitions from the freebore to the rifling. Unlike the freebore, the print doesn't seem to give a direct dimension for this 'throat' region, but does reference both to the bolt face.

When someone says their chamber has a 0.170" throat... which dimension are they referring to - or some amalgamation of the two?

 

[TonyR]

Okay... time to back up a little and clarify something.

When I look at a reamer print, say for a .308 Obermeyer chamber, I see a section labeled 'lead' (I've also seen it labeled 'freebore') with a dimension like 0.085" between two points, and a section labeled 'throat' where it transitions from the freebore to the rifling. Unlike the freebore, the print doesn't seem to give a direct dimension for this 'throat' region, but does reference both to the bolt face.

When someone says their chamber has a 0.170" throat... which dimension are they referring to - or some amalgamation of the two?

I believe that the dimension of the throat would be determined by the throat angle which is specified on the reamer print. So, if you know the free bore and the throat angle, you have what you need. I do find it hard to figure the freebore from the dimensions I have seen on some reamer prints so I prefer to give the smith a dummy round with the bullet seated where I want it then ask to have the reamer ground so that the dummy round bullet would touch the lands. That has worked out pretty well so far.

 

[memilanuk]

BTT...

 

[scaxeman]

Personally, I'd just wait till the Team reamer got in...

 

[memilanuk]

Not much help you are... :

 

[Gundoktr]

TonyR has the right answer for you. Like i mentioned before, you have an Obermeyer spec reamer, it has a .3085" diameter neck and .085" of freebore before the leade(the taper to the rifling from the freebore). You must have some idea of the bullet you are going to use! Why not just load a dummy case or two with that bullet seated to where you think it wil shoot best and have someone (me) throat your barrel so the dummy round will allow the bullet to just touch or give you a specified "jump" to the rifling(leade). Unless dyou are going to buy a spec reamer with those custom dimensions, you are trying to put the cart in from of the horse.

 

[memilanuk]

What scaxeman was saying was kind of an inside joke... and even if it isn't, I don't want someone to just magically 'make it right' for me, I want to understand *why* things are the way they are for this matter. I can take dummy rounds to any local 'smith and have things made to fit those particular bullets. Thats not what I'm after.

The problem is somewhat moot at this point, as the 2013 US F/TR reamer is all blessed and official and such - and I have one on order. Between one thing and another, this is the option I'm going with for now. I am still curious about the 'how' and 'why' for future reference, though.

I still think there must be some ambiguity in the term 'throat' as commonly used. For example, one person I corresponded with has a chamber cut for him with a specified 0.170" 'throat'... while another has a chamber cut with (essentially) the reamer mentioned above, which the PT&G print lists as having a 0.170" 'leade'... and there's a good 0.050" difference in the resulting average OAL, both with B185 Juggernaut bullets.

Even taking into consideration base-to-tip variations commonly seen in a given box, lot-to-lot differences, and the possibility of one guy having a tight (.298") bore and the other one having a regular bore... thats still an awful lot of difference between two guns that were claimed to have 0.170" 'throat'.

 

[kendog]

As I understand it, the throat dimension you are referring to is the throat datum. Typically, this dimension is taken at the .306" diameter for 30 cal (.308") bullets/chambers. The Obermeyer throat datum is at 2.1564" from the bolt face. Most, if not all of Boots' chambers have a 2 degree, 30 minute throat angle. You could relieve your throat with a lesser angle throater to seat bullets farther out. Or you could set any bullet to your chambers throat datum.
The way I understand it, you mark any bullet at .306 diameter, and seat the bullet so the mark is at the throat datum (2.1564" for the Obemeyer throat) dimension from the bolt face. You could use a micrometer locked at the dimension, or make a ring gauge bored to .306" to score a fine line on the bullet. This is where your chosen bullet will touch the lands in a new throat. Of course, this is for a new chamber. The throat datum is always moving forward as the round count goes up.
The OAL has very little to do with the throat datum measurement, and is far less accurate way to measure it.

 

[memilanuk]

Ken,

I think you may have hit on what could be the point of confusion here... I think maybe some people are referring to 0.170" 'throat' as being to the .3060" standard as you mentioned, and others are referring to the actual freebore/leade length, which would be (still) @ the freebore diameter of .3085". Its late enough I don't feel like working through the trig, but that sounds about right.

Thanks,

Monte

 

[kendog]

Did I win the Buick?? ;D

 

[memilanuk]

More like the Kia...