Chamber Specs - Optimization

nksmfamjp

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Jan 5, 2004
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Seems like what is making the new PRC rounds good is partially a smaller throat diameter. Seems like this has been as big as 0.001” oversize, but newer rounds cut this in half.

Has throat angle been optimized, especially for VLD type bullets?

Also, people talk about neck diameter of 0.001-0.002” over loaded round diameter, but most seem much more oversized like 0.011”. My 300 WSM shows 0.344” loaded round and 0.345” chamber, but my loaded round is 0.334”....even with full 0.015 necks thickness, 0.338” would be max.

Freebore seems all about getting the bullet out of the case, so that is easy enough.

I just wonder if it is worth my money to specify this level of detail or if I should accept the nominal. I get that the throating can be reworked to some extent.
 

AZShooter

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Dec 12, 2005
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Volumes have been written on these subjects. I will share what has worked for me. I strongly believe if you are making a custom rifle you should buy the reamer to suit your needs. It is a small part of the cost of getting it right.

In a nutshell the SAMMI spec chamber dimensions are made with chamber neck dimensions to accommodate all brands of brass. With some manufacturers making thicker case walls the neck must have enough space around the brass allow for bullet release.

The tighter neck dimensions are used to minimize brass stretch to help extend the life of the brass due to "working it" by firing which expands then sizing which reduces the neck dimension which makes the brass harder. The hardening will cause splits in neck not to mention different neck tension over time.

A tight necked chamber, one that is in the range of .001-.003" total release (both sides added) IMO requires neck turning to ensure the neck wall thickness is consistent. This small dimension is best left for the bench rest crowd and not for a rifle that would be used for hunting.

There are many threads on what would be proper total neck clearance for a 6.5 or larger cartridge and it is trending towards a minimum of .005" or even more. Naturally a no turn neck must have enough clearance to ensure easy bullet release with all brass being used. Some brands of brass have some pretty large variances in neck wall thicknesses.

As to throating for a VLD bullet. 1 degree 30 min has worked for me. It has worked in a 257 weatherby with just over a .100" jump to the lands and with other cartridges like a 284 win with the VLD slightly into the lands. Can't say for sure if this throat angle will work under all circumstances. BUT you can always use a Berger hybrid bullet as an alternative to a VLD.

Best way to set up a reamer is to make a dummy round that fits and feeds from your magazine and send it in to the reamer maker. IMO specifying what you want with a reamer you will own is best for your setup. I currently have 14 reamers.

I would not use a throating reamer after the chamber is made unless there was a change that you discovered after using the rifle for a while. I can understand where a gunsmith might have to use a throating reamer to comply with your specifications, his reamer might have a shorter throat length. That would be eliminated with the use of a reamer made for you.

Finally there are some threads on an issue that the 6.5 PRC and a few other cartridges have. It concerns how the fired brass is fitting in the chamber. This issue can be resolved with a slight change in the reamer's body dimension. Read this thread to understand the issue:

The PRC die "problem" | Long Range Hunting Forum
 

Alex Wheeler

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Jul 5, 2017
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Too loose is better than too tight. When designing a reamer you will cause more accuracy problems by getting a little too tight in freebore diameters and neck diameters. Even in the reamers I design for Benchrest I run a little more clearance than is standard. There is no accuracy to be found in tight neck clearance. Tight freebores will also hurt accuracy. If you want to experiment and learn, order up a bunch of stuff and try it, otherwise do not just tighten things up more than standard practice end expect accuracy to improve. Lead angles can be optimized to a specific bullet, it takes a lot of work to figure that out. But its only to that bullet. It may help or hurt the next bullet. 1.5 degree is a very good angle that seems to work well with a variety and it does not have the "weirdness" of some of the other angles.
 

nksmfamjp

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Jan 5, 2004
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1,149
When designing a reamer you will cause more accuracy problems by getting a little too tight in freebore diameters and neck diameters. Even in the reamers I design for Benchrest I run a little more clearance than is standard. There is no accuracy to be found in tight neck clearance. Tight freebores will also hurt accuracy.

Isn’t the reduction of freebore clearance the key to 6.5 creedmoor accuracy? 0.2645” vs 0.265 for a 260 Rem

Also, haven’t the short range benchrest folks been setting neck clearance at 0.001” for years with great success and accuracy.

I think I am misunderstanding your point. Always interested in learning!
 

Alex Wheeler

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Jul 5, 2017
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Nope, my 6BRA reamer has a .244" freebore diameter on it and it has broken a lot of records and won national matches in 600 and 1000 yard Benchrest. Most guys run .002-.003" neck clearance.

There may still be some guys running as tight as .001" but the trend has been to give more clearance as theres no loss in accuracy and the guns arent as finicky. When you get to tight on either of those places you get picky rifles that spit shots. The freebore diameter on the 6.5 PRC is too tight. If you get a reamer that actually has a .2645" diameter on it, after a handful of chambers you will be cutting a .2643" throat and you will see scratches all around the bullet, pressure and fliers follow. Most reamers do not measure exactly what the print says. Depending on who made the reamer, Id bet most of them are closer the high end of the spec. In the freebore the tolerance is minus zero, plus .0005". The bottom line is you do not gain accuracy by going as tight as you can but you do risk hurting it. So the trend is to stay away from that.
 

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