Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions

[Mike Matteson]

Hopefully a true expert like Alex Wheeler will respond, but it seems that with modern chamber designs and their tighter tolerances, getting a specific jump to the lands of the barrel is no longer seen as important as it once was. Erik Cortina, one of todays top competition shooters, made a YouTube video entitled: "Chasing the lands is STUPID! Don't to it." The current trend is to just find a combination that works well with a particular rifle/powder/bullet combination and not worry about the length of the jump.

The Weatherby Magnums (and most recent cartridges) use a tube of snug-fitting freebore before the tapered "forcing cone" into the rifling. Some cartridges like the 30-06 or 300 Win Mag just have a cone that tends to be larger at the mouth than the freebore sections for most modern cartridges. The 300 Win Mag's cone, for example starts at .315"dia. The 300 Weatherby mag, in comparison used a .361" long section of snug freebore tube ( 0.3084" dia) to allow the bullet to travel a little farther before hitting the lands and slowing down as the rifling got engraved. This resulted in higher velocity, but required pretty tight tolerances by 1940's standards. Unlike the larger cone of a 300 Win Mag, the tight tolerance on the freebore section of the Weatherby holds the bullet straight as the rifling is engraved. FWIW, the recent 6.5-300 Weatherby Magnum has about half the length of freebore as the 1940's ones, so it seems Weatherby has decided it had too much of a good thing.

The 25-06 is an interesting case, since when it was standardized by SAAMI in 1969, it had what was considered an unusually long and tight section of freebore ( 0.105" long with only 0.0005" clearance). It has the case body of the 30-06, but a freebore/throat design more related to the .257 Weatherby Magnum. The trend for the majority of cartridges released in the past 50 years has been to have a tube of freebore with a thousandth of an inch (or even half of that) clearance around the bullet. This holds the bullet in line with the center of the bore while the rifling is engraved. This had been done with "match chambers" for competition rifles long before it became common with hunting cartridges, so the term "match chamber design" is often used for the 25-06 and many newer cartridges. Today's better tooling and machining processes makes holding these tight tolerances much easier.

The older chamber designs with only an oversized cone for the throat rely on the case to align the bullet with the bore. For a number of reasons, this can allow the bullet to be angled slightly when the rifling is engraved on it. The result is that the rifling is not engraved evenly and the bullet will tend to be cocked very slightly sideways as it spins down the barrel. On some pictures of recovered bullets, it is possible to see this uneven depth of rifling on some of the grooves. When the bullet leaves the confines of the barrel, the forces from the rapid spinning of the misaligned bullet can cause it to jump slightly sideways.

INTERESTING! I have listen to Erik on lands. He did note that as his groups open up he increase his length. He wouldn't set all his bullets to a set demension, by leaving them long and adjust as things changed.
That was one of the reason I started to purchase my reamers instead having the smith use his. The other is I want my neck area inside the chamber tighter than what factory builds them at.
The other is I was shooting a 308NM chamber rifle. Cases were starting to separate in about 3 firing. So I change out to a neck sizing die in a 300WM. It only sized about 2/3 to 3/4 of the neck in the 308NM. It stopped the case separation and improved the grouping. The case neck were cut for thickness so I feel the bullets were lined up with the lands.

 

[Batsin]

308 reamers vary somewhat. The SAAMI 308 has a section of freebore, but the dimension of a min-spec SAAMI reamer is 0.310"diameter. That is considered sloppy these days. Most of the match reamers I have seen over the last 30 years have usually had a .3085" freebore, but its length varies from design to design, depending on which bullets are to be used and how far out of the case they will be seated. It is common now for people to seat the bullet out far enough for a little of the full-diameter shank to be inside the freebore section of the chamber. That way, the cartridge is positioned by the bullet instead of the case.

 

[SheepShapeAK]

@Justice1327 It happens when you build good rifles.

 

[SheepShapeAK]

INTERESTING! I have listen to Erik on lands. He did note that as his groups open up he increase his length. He wouldn't set all his bullets to a set demension, by leaving them long and adjust as things changed.
That was one of the reason I started to purchase my reamers instead having the smith use his. The other is I want my neck area inside the chamber tighter than what factory builds them at.
The other is I was shooting a 308NM chamber rifle. Cases were starting to separate in about 3 firing. So I change out to a neck sizing die in a 300WM. It only sized about 2/3 to 3/4 of the neck in the 308NM. It stopped the case separation and improved the grouping. The case neck were cut for thickness so I feel the bullets were lined up with the lands.

Erik also runs a tuner, which is why he doesn't worry very much about seating depth. He just changes the setting on the tuner.

 

[SheepShapeAK]

Generally over several years, when I purchase a rifle, I have in mind to change out the barrel at the start. Even if it's a new rifle. More so for the action, and that's about it. I generally hand load my ammo except 5.56, and it's a m-4 type. There isn't a caliber thatI can't reload for. I don't use it, it just put away for a rainy day. I like my rifles to have at least a 26" barrel or more. Now I have them threaded for a suppressor or muzzle break.
I haven't found where I need a custom action either. I don't shoot match, and generally do quite well in getting my rifles to group. I have even gotten to having a 2nd rifle action and barrels to fireform my brass on.
What does concerns me is the jump or free bore with the copper bullets. I don't have much info. It seem to me the copper bullets like the jump. The longer bullets seem to require more room to allow for the jump. The action part isn't the problem. I never like the WBY mag, because of the free bore in it. I have shot a few of them, but it seem to me they didn't group that good. Never reloaded for one. I do have several belted mag and do just fine with the grouping with them.
Not having much experience with free bore or needs for longer all copper bullets. It's my understanding that the reamers can be changed or recut to change for more free bore and require sharpening. (NOT A GUNSMITH) Aways asking question, and learning, sometime slowly.

I can't imagine the older belted cartridges will shoot the longer higher bc solid bullets very well. Not to mention the higher twist rate that is required for them compared to what came standard in most of those cartridges.

 

[VinceMule]

7 mag does very well with the 175-180g Bullets, not sure about that weight in solids where bullets are a country mile long. There is .4g less capacity in the 7 PRC.

 

[Mike Matteson]

I can't imagine the older belted cartridges will shoot the longer higher bc solid bullets very well. Not to mention the higher twist rate that is required for them compared to what came standard in most of those cartridges.

I don't see where that would be much of problem, only the twist rate would need to go up. probable would need to open up the magazine are and change to a clip type to allow for addtiional room for the long bullets.

 

[VinceMule]

Wyatt's Internal mag boxes go to 4.000".

 

[bigbull]

Two questions really:
1) What, if any, difference is there between a reamer dimension and the hole it cuts when making the chamber...........

2) I noticed that chamber depth dimensions are sometimes slightly shorter than the max case dimensions................
Thanks

This a very good question, there are lots of opinions and here is mine.
The beginning of any custom rifle starts with the brass that will be used to feed the rifle.
1. The dimensions of the brass will guide the choices later on for the dies and the chamber reamers.
2. The dimension of the reamer has to accommodate the brass without creating excessive stretching after firing.
3. The dimensions of the dies have to resize the brass without excessive reworking of the brass.
The choices have to be made in this order so that there are no compromises along the path to your chosen caliber for your custom build otherwise it is all SAAMI spec. and good luck with that.
BB

 

[SheepShapeAK]

7 mag does very well with the 175-180g Bullets, not sure about that weight in solids where bullets are a country mile long. There is .4g less capacity in the 7 PRC.

I've got some 195 grain bulldozers. I should just put my 300wm together and see how they do. It is a proof prefit barrel, so if I got 1 MOA out of it I'd be impressed.

 

[Mike D Texas]

I can't imagine the older belted cartridges will shoot the longer higher bc solid bullets very well. Not to mention the higher twist rate that is required for them compared to what came standard in most of those cartridges.

Why would the powder vessel have any impact on the ability to shoot different bullets?

 

[SheepShapeAK]

Why would the powder vessel have any impact on the ability to shoot different bullets?

I guess the vessel wouldn't really come into play as much as the twist rate. After looking at the reamer prints for a 300WM and PRC, it appears both have .308" of freebore for SAAMI spec reamers.

 

[Mike Matteson]

It is not a matter of changing tolerances. It is a simple matter of eliminating overlap. The specs and tolerances are written on each drawing and are different for each individual cartridge. It turns out that this was done for the .30 Remington and its family as far back as 1905 to prevent the very same issues. Those cartridges were designed for the Remington Autoloading Rifle (later, the Model 8) and someone had the foresight to notice the potential problem. It seems that someone at SAAMI just forgot their own history. Heck, I had to chase down a copy of a 1992 SAAMI publication to even find the 30 Rem drawing.

Edited to add: I did not know this when this thread was started. It occurred to me to look at the 30 Rem when I was looking into early auto-loading rifles.

This a very good question, there are lots of opinions and here is mine.
The beginning of any custom rifle starts with the brass that will be used to feed the rifle.
1. The dimensions of the brass will guide the choices later on for the dies and the chamber reamers.
2. The dimension of the reamer has to accommodate the brass without creating excessive stretching after firing.
3. The dimensions of the dies have to resize the brass without excessive reworking of the brass.
The choices have to be made in this order so that there are no compromises along the path to your chosen caliber for your custom build otherwise it is all SAAMI spec. and good luck with that.
BB

Brass is what a manufactor makes. You have to determine what you want with the reamer. I generally don't change the case length, but reduce the chamber in the neck area. Most of my cases I am going to AI shoulders. I reduces my neck thickness to .013". So most case I cut to reduce the thickness. As far as chamber length I don't change it. My case won't fit into the chamber. I do fireform the cases, by reducing the powder load and extend the bullet into the lands. This in turns push the case base back into the bolt face. That allow the shoulder and neck area extend to fill the chamber. Reducing the case stretch at the bases.
I cut the necks from brand new cases. I figure I don't need a mandrel to push the irregularity to the outside of the neck. if the case have been fired, I full size them and use a mandrel to puch the irregularity to the outside to cut the necks to do two things. Clean up the thickness and make the neck even inside and out. After that I use a bushing die to set my neck tension. Working the brass less, or I feel I am doing that. Others don't agree, but that's me and them. To me their a lot of work that goes into a case before firing it, but onces done its down to annealing and sizing, checking for case length. Again I set the neck tension by bushing not using a mandrel.

 

[VinceMule]

Mike, many of the very best 1000 yd and beyond shooters, get straighter seated ammo by running a mandrel through the neck after the Bushing size. This is why Redding puts a bushing on their Comp seater busing dies. Which mandrel to use gets complicated, along with the size of the mandrel. The bushings often introduce run out on a seated round, how anal you want to get is the question.

I have been using bushing dies for 40 years, and I am very careful to check the run-out with a run-out gauge. As you have probably found, the thicker necks may end up having more run out....it is trial and error. PMA makes a great mandrel system that you can order in .0005 increments with the mandrels running around $30 each. The long mandrel straightens the crooked neck. There may be diminishing returns as you strive you get below .002 and run out.

I copied a system used by a fellow that tests bullets for Berger at 1000 yds. which is the straightest ammo that I have ever checked the run out on. The Forster factory-honed per neck dia I order dies, followed by a Mandral using turned and non-turned necks. Run out with this system is less than .001. Again, I don't know if there is any practical improvement in groups if you go below .002 which is accomplished by several methods. Bullets with 006 run out seem to create flyers, I use those for fouling shots.