deciding on the need of custom chambering


Well-Known Member
Apr 18, 2018
This is more of what throat lead should be as this isn't a regular/standard chamber. Knowing what your cartridge length restriction is and what possible projectiles are being used, what and how far should you figure minimum jump could/should be? Without getting another barrel after the jump is to far and no way to shorten it, what could be an easy/short answer? I'm going by ogive length for comparing the possible choices with best coal not being more than 2.830". With some bullets being in the oal shorter than 60-90thou, but not knowing the ogive length. If it were to be 60thou and the lighter bullets may being jumped 150+ (not necessarily an issue, but might not be accurate).
I am a proponent of owning a custom reamer. The process isn't too hard. Make a dummy round with the intended bullet that fits and feed from the magazine. Talk to the reamer maker about the specs. Send the dummy to the reamer maker.

As to particulars, a throat angle of 1 1/2 degree is common these days. It tends to work well with the high BC bullets as well as the old standards. Another consideration is the chamber neck diameter. SAMMI dimensions are large enough to accommodate all thicknesses of brass. Your particular brand of brass could have a thinner neck. A smaller chamber neck dimension would allow bullet release without over working the brass. An even tighter neck dimension can be ordered if you intend to turn the necks for the same reason. More recently there are a few chamberings like the 6.5 PRC that should have the proper dimensions near the web. You can read about that if you look up Alex Wheeler's posts on the subject.

There was a time when no one, except perhaps bench rest shooters, worried about a bullet touching or slightly into the lands until VLD bullets came along. For many shooters this was the only way to get VLDs to shoot. For some reason this concept transferred to all bullets. There are many bullets that shoot well with a wide variety of distances from the lands. The Berger hybrid bullets are a more recent design but Nosler, Hornady, and Speer bullets have been seated away from the lands for decades with many rifles shooting very accurately. Barnes recommends .050" from the lands as the starting point.

Rifles with Weatherby chambers are known for some large jumps to the lands. Many are .375" and they shoot accurately.

I have 15 reamers. I'll share some details on one of the more unusual ones. It is a 284 Win. It is in a short action with a custom magazine allowing the total length of a loaded round to be 3.045" I wanted to turn the Winchester brass because Win brass is notorious for varying neck thickness. I also wanted the VLD to engage the lands. Here is my request to the reamer maker:
I am requesting a 284 win chamber reamer with a .315" neck and a 1 1/2 degree throat angle. Also want a removeable pilot bushing installed. Use the supplied dummy round to throat reamer for the bullet to engage the rifling with .020" engagement. (if that isn't possible then to kiss the lands)

The reamer works and the 168 hunting VLDs fit and feed from the magazine with excellent accuracy. I also wanted to shoot the 120 Nosler ballistic tips. They are not touching the lands. They also fit and feed well and shoot accurately with a jump to the lands.

You can talk to your gunsmith about his reamer's specs which might work for you or order your own reamer.

Hope this helps. I am certain others will offer their thoughts as well.
This thread ties into another thread I posted about 6.5prc. This is the specific cartridge that'll be used for an ar10 build an is why the mag limit length. Up to 135gr (except lapua 144fmj possibility) will be used. Making dummy rounds for all possible bullets in question and compare ctbo lengths of each for possible throat lengths. All though I do have a uni-throater I would start with their dimensions. I have 6.5creed and 260rem in ar10's, but just wanted something with a little more velocity. If I'm starting out with max oal there's a possibility of shortening oal, but can't shorten throat if needed. If I'm down to 3 or 4 bullet choices due to accuracy not working for the others, then that's fine. I just don't want the throat to be toooooooo long and render the barrel useless. What might be a good starting point for throat length without breaking out the uni-throater more than twice?