Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions

[Bob Wright]

SAMMI is 'voluntary' compliance for the production gun industry, with a lot of tolerance.
However, I've had match reamer "no turn necks" done that required every piece of brass to be neck turned. Wild vertical stringing and ridiculous ES started me digging into why.
Nothing is as it seems to be stated with my luck.

 

[VinceMule]

European brass is often larger in the web and neck thickness can vary greatly between different brands. If you use a SAAMI spec reamer, then try to use Lapua brass in some calibers, it can be problematic, 243 Winchester brass has a tremendous spread between Winchester and Lapua brands, and guys going to the 243 AI have been busted hard on this issue, the same issue applies with 223 Remington and 22/250 brass. I am referring to custom barrels not sloppy factory chambers.

Some of the very best gunsmiths spend a lot of money, perfecting their Reamer designs for accuracy and ease of use. This always involves multiple reamers that have been ordered to make subtle changes. This type of information on Reamer design is often the Heart of a gunsmith's success, and they are not apt to just hand out their Proprietary reamer designs. I would not fault a gunsmith for not handing out reamer prints with all the time and money that they have invested in very successful, match-winning reamer designs.

 

[ImBillT]

Sloppily indicating the barrel, and using insufficient distance between the two points that are being held are big reasons for fat chambers. Another is not having your reamer exactly on the center line of the barrel. If you lathe weighs or tooling are worn this is possible. Some guys don't check. I assume a floating reamer holder cures that for the most part.

As mentioned, the reamers don't always come out exactly to spec. It's actually pretty hard to make something perfect, so there's a tolerance. That varies from reamer maker to reamer maker. PT&G has tighter tolerances on their "match" reamers than their regular SAAMI reamers.

The SAAMI specs for reamers and chambers are so that everything from every manufacturer(that is within the spec) will function and function safely. A .004" crush fit of the smallest chamber and largest case will still chamber with some effort. A .006" space between the largest chamber and smallest case will still function safely, but potentially cost brass life. If you're having a rifle built for maximum accuracy, you may not want to use an off the shelf SAMMI reamer, because it would be any dimension within the spec, but you also may not want a SAAMI min spec reamer that was held to very tight tolerances unless you have settled on what brass you're going to use and know what size it is consistently. Many shooters have customs spec's reamers to be a close fit to specific brass. There are reamers cut for "Gold Box" Lapua, "Blue Box" Lapua, Norma, "White Box Winchester", Winchester Palma brass, Remington BR brass, and likely others. These reamers give proper minimal clearance for those batches of brass and work very well without fire forming, and when paired with a custom sizer they give excellent brass life and excellent accuracy right out the gate. However, they may be unsafe and yield reduced accuracy with other brass or off the shelf dies. Lapua Gold Box was a smaller dimension than Lapua Blue Box, and if you have a chamber reamed specifically for Gold Box, you will have difficulty using Blue Box because the web is too large for the chamber and sizing that down is extremely difficult. IIRC Blue Box is close to SAAMI max case dimensions, which, when used in factory chambers of SAAMI specs results in better than average accuracy from virgin cases as well as less brass growth. Gold Box was closer SAAMI min case specs which resulted in quite a loose fit on factory chambers. Not only was accuracy likely degraded with virgin brass, but brass life was probably also degraded due to excessive case stretch on the first firing. If you had a reamer designed for Gold Box brass, then it was absolutely excellent brass.

 

[ImBillT]

SAMMI is 'voluntary' compliance for the production gun industry, with a lot of tolerance.
However, I've had match reamer "no turn necks" done that required every piece of brass to be neck turned. Wild vertical stringing and ridiculous ES started me digging into why.
Nothing is as it seems to be stated with my luck.

While that could easily happen, I still have to ask. Was it a "no turn" for the specific brass you were using? I feel like you could get a no turn reamer for Lapua Blue Box, and a year later get a lot of Blue Box that had thicker necks and didn't work, so I'm not arguing at all. I'm just asking because if you had a reamer that was no turn for X brand, it is by no means guaranteed to work with Y brand. That may not be clear to everyone.

 

[VinceMule]

Quote, "if you had a reamer that was no turn for X brand, it is by no means guaranteed to work with Y brand. That may not be clear to everyone". Gospel Truth here, and almost guaranteed to have a difference in Web dimension also.

 

[ChrisInKY]

Awesome knowledge on these pages.

 

[Mike Matteson]

A. Reamer makers have .0005 tollerance on a print vs actual grind dimension. This can be + or -, so you have to do some measuring when you get the barrel back from the gunsmith. Making a chamber cast with Cerrosafe tells all.

Also, chambering techniques vary which will have a great influence on how close to reamer spec the chamber is cut.

Reamers wear with use. Naturally, the reamer can get smaller as cutting flutes wear. Again, chambering techniques have a GREAT influence on reamer wear. Various cutting fluids vary dramatically in how they reduce friction/heat.

Speedy told me that a guy that taper bores his chambers will get 150+ chambers on a reamer due to only cutting .007 per side with the reamer.

Your question, "Does a typical reamer actually need to be slightly larger or smaller than the finished hole diameter?" I hope the above answers your question. I order reamers to the dimensions that I want from JGS and Manson with great success.

B. Chamber lengths are determined by the gauge that the gunsmith uses to set the headspace. There is the Go, No Go, and Field. There is somewhere around .003-.004 difference between the Go Gauge and the No go gauge. This could explain some of the chamber lengths.

It is interesting to determine the "spread" in headspace lengths of 100 new pieces of brass which is simple to do with an oversize ogive gauge. I have had gunsmiths use the new brass to set the headspace of my rifle, but I did determine the longest and shortest headspace lengths. When you are using the shortest Headspace length to eliminate any case stretching, you may have to bump the shoulder on the longest or have a very stiff bolt close when fire forming on your first firing. You could ask the gunsmith for a headspace length where the bolt barely closes on the Go gauge to eliminate case stretch. Some gunsmiths will accommodate you and some absolutely will not.

New brass dimensions can vary greatly from manufacturer to manufacturer. I have found that Winchester has the smallest dimensions in the Web while European-made brass can be much larger, and neck thickness is all over the map also.

That's what I see with brass, is the thickness can vary in the neck, and other areas too. That's why I cut all my new brass neck to a thickness too start with. New or unfired brass right out of the box, Fired brass I full sized them. Use a mandrel to size the inside of the cases and then cut to thickness. Depending on thickness of the necks. It may take 2 pass in cutting the thickness I should say a 2nd setup to final the thickness cut. I messed a step in there. I cut all my case to length to start with. They way my cutting for thickness all stop at the same place. Consistence make for accuratic.

 

[Taj]

I wouldn't pay no mind to anything that comes off that guys YouTube channel.

There's been a lot of comments that dude has made that's had people wondering on this forum and a few others. If ever there was a fuddlore origin, that would done be it.

I'm with you on that. I've been building rifles for over 50 years and some of the stuff I have seen on that channel makes me cringe. Gunsmiths are sort of like doctors. There are some really good ones (Technicians) and then there are the other guys.

 

[Batsin]

The SAAMI specs for reamers and chambers are so that everything from every manufacturer will function and function safely. A .004" crush fit of the smallest chamber and largest case will still chamber with some effort.

This is an area that causes problems with the current SAAMI specs, though. The assumption is that every rifle can crush fit the case into battery whereas many, especially autoloaders, cannot consistently do so and operate as designed. That is why I have been surprised that this crush fit dimension has been left in for cartridges that have been designed specifically around the AR15 platform, for instance. A very small change in the specifications would avoid this problem.
Cartridges like the 6.5 Grendel and the newer 22 and 6mm ARC were designed around the magazine length and bolt-strength limitations of the AR15 platform. The SAAMI max chamber pressure of 52,000psi (transducer) reflects that. It seems odd, then, that other tolerances were not also adapted. An in-spec case can still require a crush fit into an in-spec chamber.

FWIW, I re-read my own posts and realized that a couple of things were not clear. The headspace dimensions in post #1 are for the 308 family.
In post #3, I intended to convey that the new 300 Norma Mag and 300 PRC spec bullets SMALLER the traditional dimension. The max bullet diameter for the 300 NM, for example, is called out in the SAAMI drawing as 0.3083" (same as the Weatherby Magnum) vs .3090" for most other .30 cal cartridges.

 

[ImBillT]

You have to remember than SAAMI doesn't control manufacturers, it simply puts out guidelines. An intelligent manufacturer of an AR etc. can apply any in house spec they want. There's no reason than an AR manufacturer cannot simply spec their chambers to be .000" to + .006 rather than -.004 to +.006". As long as the brass falls within SAAMI spec, the rifle manufacturer should be able to make a chamber that will work, even if they had to hold to a tighter tolerance than SAAMI allows due to their rifle design. If your autoloader doesn't work with ammo that is within SAAMI spec, it's the rifle manufacturer's fault, not SAAMI's fault.

If SAAMI tightened the tolerance, then EVERYONE would need to abide by the tighter tolerance, even if it wasn't needed for their design, and then costs would go up for everyone. There is a reason that Lapua brass costs more than Winchester, and there's a reason than a rifle from Alex Wheeler costs more than a Savage.

 

[Bob Wright]

While that could easily happen, I still have to ask. Was it a "no turn" for the specific brass you were using? I feel like you could get a no turn reamer for Lapua Blue Box, and a year later get a lot of Blue Box that had thicker necks and didn't work, so I'm not arguing at all. I'm just asking because if you had a reamer that was no turn for X brand, it is by no means guaranteed to work with Y brand. That may not be clear to everyone.

Yes, agree. Generally we have no control over how things are made. We may have to "correct" some discrepancy when we buy something.
The gunsmith was telling me I was loading them wrong (he himself did not reload) and when I showed him the necks of 2 different brass vs. the chamber, he wouldn't accept the evidence. It was simple arithmetic. It was never a "no turn" situation here. He told me I couldn't shoot a solid mono bullet because it was "impossible" to function like a cup and core (obturation).
Haven't found my new smith yet.

 

[crazymonkey]

Being a machinist not a gunsmith by any means . I can tell you that sometimes it's about setup as .001 runout is .002 on the diameter it's cutting. We get alot of new form tools ( readers with multiple stages of dia or angle) that will not cut for poop. It's about how well the entire thing is done setup,runout,and quality tools . Had to indicate a .2501 reamer to cut a hole .250 to .2502 that takes a indicator in .00005 to do .

 

[59FLH]

Two questions really:
1) What, if any, difference is there between a reamer dimension and the hole it cuts when making the chamber. The Real Gunsmith mentioned this once but did not elaborate. For example, if the diameter spec for the freebore section of cartridge (6.5 Creedmoor, for example) is labeled 0.2645", what is the actual dimension on a reamer needed to make that hole exactly 0.2645"? I understand that there is a max tolerance of + 0.002", so the hole can be slightly larger and still be in spec, but the question is: Does a typical reamer actually need to be slightly larger or smaller than the finished hole diameter?

2) I noticed that chamber depth dimensions are sometimes slightly shorter than the max case dimensions. For example, the headspace diameter dimension of 0.400" is 1.630"(min) - 1.640"(max) from the breech bolt face on the SAAMI drawing, but the max case dimension at 0.400" dia is 1.634" from the base of the case. In other words, it appears that a max-spec case will be a crush fit into a min-spec chamber. I can understand that the distance from the breech face to the the shoulder-neck transition, for example, could be a touch shorter since the chamber is slightly larger than the case there, but a min-spec chamber being smaller than max-spec case at the headspace ring has me puzzled.
Thanks

To answer your question(s) if I understand them correctly.

1) In general, reamers by design should cut to size. meaning a .1875" diameter reamer should cut a hole .1875" in diameter. Assuming its ground correctly and sharp, how the reamer is used (rotational speed, chip load, feed rate, peck or single pass, run out, etc) can influence final hole size.

2) Your observation of .004" case to chamber crush fit at the HS dimension is correct, not sure what the question was.

 

[Batsin]

You have to remember than SAAMI doesn't control manufacturers, it simply puts out guidelines.

Correct. Just put out guidelines that work better. Nothing needs to change on the older cartridges. New ones just won't have the same problems. Changing the overlap on tolerances does not require making the parts more accurately, it just means eliminating the overlap in the initial specifications.