Barrel Throating

[Tikka]

I recently came across a new term (Well new to me)

 

Can anyone explain to me about barrel throating? (In laymans language).

 

If a barrel is throated for a particular bullet, can I expect to get good accuracy from

other weight bullets?

[Vermincinerator]

Tikka,

The throat is the distance from the chamber neck portion to the lead-in onto the lands, also called freebore and will be bullet diameter or a tadge more.

SAAMI spec chambers will have a throat cut to a depth to accomadate all bullets which are available in factory loaded ammunition for a given calibre, a sort of compromise.

No factory ammo will have bullets seated that will jam into the lands, this is the domain of the handloader.

 

With a custom chamber a smith will spec a reamer from the makers with a specified throat length for a certain bullet his customer wants to use.

Alternatively, he may use a reamer with a very short or even no throat and cut the throat to the specified depth with a throating tool.

Speccing a certain throat depth for a chosen bullet will allow you to use bullets which in a SAAMI spec chamber would need to be seated to far into the case to enable it to be chambered, taking up valuable powder space,

 

The longer throat lets you seat the bullet in the neck utilising all of the case body for powder charge.

IMO speccing a throat for a certain bullet can be a little risky especially if your new custom tube does not shoot well with that bullet.

As for the accuracy effect on other bullets, that can only be found out by trying different types and styles of bullet and recording your results.

 

Ian.

[Andrew]

Throating involves cutting the area ahead of the chamber to fit the bullet. These can be to standard spec or custom cut for length, angle, and diameter. Whether or not you get "good" accuracy is highly dependent on your definition of "good". If we're talking about a hunting rifle, yes. A high grade bench rest rifle"? Maybe. ~Andrew

[Ronin]

Just to add to the informative posts, a normal chamber reamer will cut a short lead (throat), there are available specialist throating reamers which have a 1.5 degree angle of cut (from parallel), this allows a gunsmith, who has cut your custom chamber, to insert the throating reamer up to the end of the existing throat and extend it (lengthen) so you can seat longer heavier bullets.

 

An example of this application would be a chamber cut for a 6mm case which is designed for 80g bullets.

 

The shooter wants to use 105grainers but finds that case capacity is diminished considerably when he uses them in the existing chamber.

 

Gunsmith uses 6mm throating reamer to re cut the throat only back a few thou so that the bullets can be seated at a point where the case may be filled to the correct amount of powder.

 

In reality, yes its benchrest and F Class shooters who would use this unless you are using heavy for calibre bullets.

 

Its alos lot easier to make up a dummy round before once you have decided on calibre, seat the bullet to the base of the neck (optimum place for powder density) and send dummy to a reamer manufacturer so they can make the reamer to suit your cartridge and bullet seating dimensions (does not cost any more either)

 

If you are planning a custom build and want to use one type of bullet, this is the way to go.

 

(well its what I do and have just done for a 6.5x47 - with a few other mods to the "normal design")

[silentsoulsleave308holes]

The longer throat lets you seat the bullet in the neck utilising all of the case body for powder charge.

IMO speccing a throat for a certain bullet can be a little risky especially if your new custom tube does not shoot well with that bullet.

As for the accuracy effect on other bullets, that can only be found out by trying different types and styles of bullet and recording your results.

 

Ian.

 

 

Ian is quite right and when I looked at getting a new barrel I looked into the pro's and cons of this and the interesting thing with this is that I actually went the other way just to be different (you know me, have to be awkward)

 

I worked out that from my previous barrel which was a 26" one in ten twist I got x velocity with x powder and this was optimum for that twist so when ordering my new barrel I run the math as to what I would get from the same load/bullet from a 32" barrel

 

it turned out that with the increased velocity gained from the extra length I could drop the charge of powder right back to saami's minimum load and get top velocity that matched the twist

 

based on this I wanted a seated bullet to sit ontop of a charge of powder to give 95-100% fill (gotta have a little +&-) so that there is less variable's but also wanted to be touching the lands

 

the result was a custom chamber min spec and the throat was cut based on real time tests and calculated gaines, the bullet would be sat back in the case(near saami length to give you some idea) just touching the powder so giving a full fill and the bullet would be touching the lands

 

on reciept of the new barreled gun I took it to the field with bench and all the other crap and reloading gear, I loaded the very first round at the minimum that the books say and what myself and the gunmaker had agreed on and bang..........................looked at the crono and got exactly the velocity that I wanted for my ten twist 2850 fps from 210gr berger and havent had to changed a thing since

 

there is so so much that I can chase the lands out with and the barrel is so so long that it should last a lot lot longer than they normally would and the decrease in charge gives extra life too (lesss heat, less pressure)

[jon]

there is an article on this at ... www.bellmtcs.com/FAQ/ChamberThroats101.htm

sort of angels on a pimhead accuracy!

jon