First of all I hope this isn't too dumb of a question - I'm really trying to better understand how things actually work when it comes to chambering a rifle.
I know they all go "Bang" when you pull the trigger but I'm trying to learn what makes things more accurate.

Anyway, here is my question and it concerns having a custom barrel installed on a rifle, in this case it would be on a model 700 short action.

So I think I get the chamber part and its need for being concentric to the rifle bore and so on, but where I'm confused is on the throat area of a rifle chamber. Is it correct that this should be keep to as tight a dimentions as possible to make it more accurate?? Or is that a bad idea??

I guess what I'm asking is this, can a new custom barrel be chambered/throated to best perform with factory ammo? I know that with handloading you can always "work up" a specific load to be more accurate, but is it possible to chamber a barrel to be really accurate for most factory ammo - or is that impossible to do the differences in factory ammo??

Again, I apologize if this is a really dumb question to ask.

Think of throat as being the "birth canal" of the barrel. What happens there, will have a huge influence on the path the bullet takes over it's short life. If a bullet hops, skips and jumps from the case to the bore, then there is a good chance that it will enter the bore, off center, with regards to it's natural axis. The bullet forms to the bore in this position as spins towards the muzzle. Once the bullet exits, it becomes conflicted between two different axes, the natural and the false. The two struggle all the way to the target. This certainly affects overall accuracy.

Some folks, in an effort to improve accuracy, will purposely seat their bullets out into the lands. A lot of them do so without really understanding the mechanics of the problem. Some folks fireform and neck size their stuff in an effort to improve accuracy without really understanding what it does. The mechanics behind that is to reduce the amount the case shifts when chambered, which can have a direct bearing on the alignment of the bullet with the bore.

For the most part, I machine the throats separately. The throat reamers I use are .0005 over bullet with a one and a half degree lead. This, along with a good set up, assures as straight a path as possible, for the bullet to travel from the case to the bore. It's amazing how much consistency is affected by the throat.

Ok, thank you for that explanation. I think I'm understanding that - the throat relative to the chamber and bore of the barrel.

So when you machine the throat in your example you give, will the dimentions you end up with shoot "most" bullets accurately?

I may be asking for an impossible solution to what is probably a more complex process. Can you chamber/throat a barrel so that its accurate firing a variety of bullets - as long as they are of the same weight?? Or is that too much of a wildcard with any given barrel??

Custom barrel + factory ammo = why bother?

I know I know - gunsmiths probably all shake their head at this question, but trust me I shouldn't be trusted around loose gunpowder-grin

Yes, a custom rifle can be throated/chambered to shoot factory ammo & then pretty much shoot 1 hole......ask me how I know??

300 WM - throated/chambered for Federal Premium 180 grain Nosler Accubonds:




.308 - throated /chambered for 168 grain Federal Gold Medal Match:

Handloading is a simple process. If you can read basic english and operate simple tools, then you can handload.

I agree Drew but I have just never got into it - maybe someday.......

Malm:

How do you determine the correct depth (length) of the throat.

I have throated a few rifles that were chambered without a throat on the reamer. I could not find any information on how to determine proper throat length. Some sources said to throat for the bullet you are going to use, but did not offer any advice on how to throat for any particular bullet.

I measured the throat depth (length) om a very accurate factory .30-06 rifle and throated to that length. IIRC, it was about 5/16 inches, Or about or a little over, one caliber.

Seat the bullet so that the base of the bullet ends where you want it in the neck. Then, throat the barrel until the bullet in your 'dummy' case just contacts the lands.

Good shootin'. -Al

Yup, just like that!

A little further explanation on how to accomplish it with minimum tools. Place the barrel vertically in the vise and drop an empty case in the chamber and take a measurement from the base of the case to the torque shoulder of the barrel. Then, insert the "dummy" round and re-measure. Advance your throat reamer the difference between the two measurements.

When customers leave it to me to determine and set the throat length to a particular bullet, without providing a dummy, I simply drop an empty case into the chamber, as in the above example, and take the measurement. Then I drop the actual bullet into the chamber where it comes to rest against the lands, and with my depth mic, measure the distance from the mouth of the chamber to the base of the bullet, and then transpose that to where the bullet would sit in the empty case to determine the exact distance needed to go to set the depth.

It's a lot less of a hassle using a seated dummy round if you can. And if the customer doesn't have any preference for seating depth, I always set the throat so that the base of the bullet is flush with the base of the neck. And if that bullet is a boat tail, unless otherwise specified, that area is the transition point where the parallel portion of the bullet ends, and the boat tail begins. Of course, whatever you do, it has to fit the magazine.

Provided the bullet weight doesn't exceed the twist rate of the barrel, all bullets within a given caliber benefit from a straight, minimum diameter throat.

Malm, what is your recommened clearance in the mag well? I have heard leave a gap between the bullet tip and the magazine of .050-.100" when making a dummy round.

Thanks.

.050 clearance is a good number to follow. That will still allow you to seat the bullet out a touch more without interference. Always leave yourself some wiggle room just in case.

I also usually throat rifles with a parallel throat which is .0005" over nominal bullet diameter (ie. a 30 caliber wil get a .3085 throat). This generally works out well but not always. Upon occasion, one may get some bullets which are a bit large and this can be a problem.
The primary requirement for the rifle's throat is that it be concentric. The 1.5 degree angle in is SOP nowdays though some still prefer the 3 degree and one reamer maker thinks a 3/4 degree might be better. Who knows?
I have shot some very accurate rifles which were chambered with the typical long, tapered, European style of throat. Shultz and Larsen target rifle barrels were chambered this way and worked real well.
If the throat is concentric and of .0005" over bullet diameter, the amount of freebore (distance to the lands)does not seem to be real critical in the world of hunting accuracy.
I still favor the throat as described by Malm but the longer I do this, the less certain I am that anything is certain.
The most accurate hunting rifle I have in my rack has a long tapered throat and the bullet is nowhere near the lands. The most accurate target rifle has a parallel throat and shoots best with the bullets well into the lands. GD