Question: Reamer Dimensions vs Chamber Dimensions vs Case Dimensions

[Batsin]

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

 

[VinceMule]

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.

 

[Batsin]

Thanks. That comment from The Real Gunsmith had me wondering, but then again he didn't cite any examples. When worked at a factory, we used precision grinding or honing when we had to hold under a thousandth tolerance.

I learned about the headspace and case issue back when the 6.8 SPC was going through its painful introduction. There were some problems with AR15's with in-spec chambers not loading in-spec ammo (there were a lot of bad reamers and undersized barrels, but that is another story). One brass maker commented that whereas the crush fit allowed in the SAAMI spec was not a big deal for most bolt-action guns, semi-autos were more sensitive. Brass had to be around min-spec to work well in AR15's.

The standard SAAMI tolerances have not changed for decades. Even on new cartridges like the 22 ARC, chamber specs are + 0.002" dia and +0.015" length. Cartridge body specs are -0.008". A lot of the accuracy of newer cartridges is attributed to long sections of tight freebore, but there is a big difference between 0.0005" and 0.0025" clearance. One is considered "match" and the other, "sloppy".
At least more cartridges are tightening up on max bullet diameter. A surprising number of cartridges allow the max spec bullet to be larger than the bore. The traditional 30 cal bullet spec, for example, has been 0.3090" - 0.003". At least most barrel throats have a fairly gentle final forcing cone. On the other hand, several cartridges now run such a tight min freebore spec that they had to tighten up on the bullet spec as well, since a 0.309 bullet will raise hell when it hits a freebore that is as small as 0.3084" diameter. The forcing cone into the freebore is often as abrupt as 45° per side. FWIW, Weatherby Magnums specified a smaller max bullet dia of 0.3083 and call it out on the SAAMI print. Now, the 300 Norma Mag and 300 PRC spec bullets under the traditional dimension.

 

[SheepShapeAK]

Glad I found this thread, as I was having the same questions in regards to the rifle I just had built. I contacted multiple reamer makers to see if their prints were the most up to date as I heard there were issues with the first reamers compared to the cases Peterson was making.

I'm wondering if next time I should send a few brand new cases for them to get measurements off of before I get a reamer made if it's a less than utilized cartridge.

 

[dfanonymous]

Thanks. That comment from The Real Gunsmith had me wondering, but then again he didn't cite any examples. When worked at a factory, we used precision grinding or honing when we had to hold under a thousandth tolerance.

I learned about the headspace and case issue back when the 6.8 SPC was going through its painful introduction. There were some problems with AR15's with in-spec chambers not loading in-spec ammo (there were a lot of bad reamers and undersized barrels, but that is another story). One brass maker commented that whereas the crush fit allowed in the SAAMI spec was not a big deal for most bolt-action guns, semi-autos were more sensitive. Brass had to be around min-spec to work well in AR15's.

The standard SAAMI tolerances have not changed for decades. Even on new cartridges like the 22 ARC, chamber specs are + 0.002" dia and +0.015" length. Cartridge body specs are -0.008". A lot of the accuracy of newer cartridges is attributed to long sections of tight freebore, but there is a big difference between 0.0005" and 0.0025" clearance. One is considered "match" and the other, "sloppy".
At least more cartridges are tightening up on max bullet diameter. A surprising number of cartridges allow the max spec bullet to be larger than the bore. The traditional 30 cal bullet spec, for example, has been 0.3090" - 0.003". At least most barrel throats have a fairly gentle final forcing cone. On the other hand, several cartridges now run such a tight min freebore spec that they had to tighten up on the bullet spec as well, since a 0.309 bullet will raise hell when it hits a freebore that is as small as 0.3084" diameter. The forcing cone into the freebore is often as abrupt as 45° per side. FWIW, Weatherby Magnums specified a smaller max bullet dia of 0.3083 and call it out on the SAAMI print. Now, the 300 Norma Mag and 300 PRC spec bullets under the traditional dimension.

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.

 

[FEENIX]

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.

This! I cannot make myself watch his videos; the comments can be entertaining, though.

 

[supersubes]

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

Ive found that reamers generally cut to the print. I dont think I have a single reamer that even comes close to the edge of tolerance listed on the print.

 

[nksmfamjp]

One thing we need to understand is YouTube and most social media are setup as a one way conversation.

Forums are 2-way group conversation.

Facebook is 2-way generally with a 1-way option.

1-way conversations can be a great source of know-how when delivered by a person who really knows the subject matter and can communicate appropriately to the audience. Think science/engineering journals or textbooks. These are all written by highly vet's and edited people. YouTube without a good internal editing process is not. Just think. I can get on there and tell folks how to tune triggers. Then blame you when they are messed up!

Two-way communication is validated by the community. I think we do a decent job of alerting folks when we get the guy who jumps on to tell us about his 156gr load from his 6.5 creed going 3000fps, zero drop to 500, elk slayer at 1000yd story. I mean how great is this all from a 6.5 lb rifle with Chinese optic shooting 1/4" 1 shot groups to a mile!

So, back to the op. It is good to cross validate social media posts. Sounds like I need to order a chamber cast with every rifle from here out. How do I determine CBTD on a chamber cast? Seems like a case fired 2-3 times without resizing the body is better??

 

[tracybartlein]

You have min spec reamers/chambers, what guys call match spec chambers and I use this phrase loosely, and you have what is called production spec chambers. A gun manufacturer can make the chambers to Saami min spec, or they can use what is called production spec. and that is up to their discretion. I've got prints for production chambers that are as big as +.002" oversize on the diameters in some cases.

How the reamer is ground and how it cuts can be two completely different things. It can cut to size, and it cannot cut to size. Dealing with this right now.... reamers are to spec for a job we are doing for ammunition test barrels, but the dam things will not cut at all! The owner bought 10 of them. Sent us two of them. You can literally roll the reamer in your fingers, and you cannot cut yourself....it won't bite at all.

Also I've seen brand new reamers where we have to measure the chamber and supply an inspection report where we have to measure the chamber in 10 different areas to check the diameter and we have to hold a tolerance on the given diameter. Reamer measured to spec but half of the dimensions cut oversize. To the point you cannot save the barrel. We scrapped $5k worth of barrels because of it (only 6 barrels). I sent the reamer back to the manufacturer all I got on the phone and by email was the reamer was in spec. I said it might measure to spec but it isn't cutting to spec! Then I sent a breech end of one of the barrels along with the gov't inspection gauges along with our inspection report and told them you measure and tell me what you get! Again, all I get told was the reamer was in spec. I said I was done...send all my stuff back. This wasn't the first time we had issues with this reamer manufacturer, and I refuse to buy anymore. If a customer sends us his action and reamer to fit up a barrel to his gun, we will take his reamer and run it into a scrap blank to see how it cuts first before we run it into a good barrel.

I helped a customer last year.... twice he sent his rifle back to the gunsmith because it wouldn't chamber box 308win ammo. The gunsmith charged him for the rework. I told him to ask the gunsmith for a copy of the print and the gunsmith refused to give him one. He gets the rifle back and again it won't chamber ammo and is sticking bullets in the barrel. Finally he called and asked if I could help as it was supposed to be our barrel. I said sure send it in. Well the gunsmith only used a roughing reamer from what we seen. Then I had to break the news to the guy that it wasn't our barrel on top of all that! We got him all fixed up and returned it to him at n/c. Gun functioned fine and actually gave him groups of 3/4 to 1moa. Which he was o.k. with as it was a hunting rifle but not what he really wanted.

My recommendation is this if you're getting a custom chamber/barrel installed. Ask the gunsmith for a copy of the reamer spec/print at least you have a reference for what was supposed to be put into the barrel.

I don't like renting tools. You don't know how many times the reamer has been used is just one reason.

Later, Frank
(Edited)

 

[VinceMule]

Good post Frank! I used that same company for decades, then things went to pot.

 

[tracybartlein]

Good post Frank! I used that same company for decades, then things went to pot.

That guys 308win gun I listed above... forgot to mention... it would headspace but still wouldn't always chamber box ammo. Some ammo did and some didn't.

So yes, you can take a roughing reamer and chamber a barrel and make it headspace but that doesn't mean it's right. Not the first time I've seen that happen.

 

[VinceMule]

If the gunsmith test fires the rifle before it leaves the shop, issues are often found. Same for examining the fresh cut chamber with the bore scope while the barrel is still in the lathe.

 

[Alex Wheeler]

If your getting a custom rifle or barrel built, it will most likely be saami minimum specs on the case body. The freebore is often specd differently based on the bullet choice. Theres no reason to tighten up the body and all it does is cause problems. We dont even do it in Benchrest contrary to popular belief. I highly recommend dealing with a good reamer maker, I use JGS. Never have I got a bad reamer and I have over 150 at this point. With JGS I expect the reamers to measure .0002"-.0003" over print specs. Which is intentional. The reamer should not cut much over that if the gunsmiths setup is good. If the setup is not good then yes it can cut oversized. The worst I have seen was a 300WM .011" oversized. At that point, he was using the reamer as a single point form tool. You can get a lot of chamber out of a reamer if you treat it right. You will never wear the body down enough to have an issue. The freebore is the first part of the reamer to wear to the point that you should replace it. A tight freebore diameter is an accuracy killer. I can tell when they are getting small with a bore scope and retire the reamer even though its still cutting perfectly fine. Theres no number on that though, I have retired some at 50 and have one that I cant kill with many 100s on it.

 

[SheepShapeAK]

You have min spec reamers/chambers, what guys call match spec chambers and I use this phrase loosely, and you have what is called production spec chambers. A gun manufacturer can make the chambers to Saami min spec, or they can use what is called production spec. and that is up to their discretion. I've got prints for production chambers that are as big as +.002" oversize on the diameters in some cases.

How the reamer is ground and how it cuts can be two completely different things. It can cut to size, and it cannot cut to size. Dealing with this right now.... reamers are to spec for a job we are doing for ammunition test barrels, but the dam things will not cut at all! The owner bought 10 of them. Sent us two of them. You can literally roll the reamer in your fingers, and you cannot cut yourself....it won't bite at all.

Also I've seen brand new reamers where we have to measure the chamber and supply an inspection report where we have to measure the chamber in 10 different areas to check the diameter and we have to hold a tolerance on the given diameter. Reamer measured to spec but half of the dimensions cut oversize. To the point you cannot save the barrel. We scrapped $5k worth of barrels because of it (only 6 barrels). I sent the reamer back to the manufacturer all I got on the phone and by email was the reamer was in spec. I said it might measure to spec but it isn't cutting to spec! Then I sent a breech end of one of the barrels along with the gov't inspection gauges along with our inspection report and told them you measure and tell me what you get! Again, all I get told was the reamer was in spec. I said I was done...send all my stuff back. This wasn't the first time we had issues with this reamer manufacturer, and I refuse to buy anymore. If a customer sends us his action and reamer to fit up a barrel to his gun, we will take his reamer and run it into a scrap blank to see how it cuts first before we run it into a good barrel.

I helped a customer last year.... twice he sent his rifle back to the gunsmith because it wouldn't chamber box 308win ammo. The gunsmith charged him for the rework. I told him to ask the gunsmith for a copy of the print and the gunsmith refused to give him one. He gets the rifle back and again it won't chamber ammo and is sticking bullets in the barrel. Finally he called and asked if I could help as it was supposed to be our barrel. I said sure send it in. Well the gunsmith only used a roughing reamer from what we seen. Then I had to break the news to the guy that it wasn't our barrel on top of all that! We got him all fixed up and returned it to him at n/c. Gun functioned fine and actually gave him groups of 3/4 to 1moa. Which he was o.k. with as it was a hunting rifle but not what he really wanted.

My recommendation is this if you're getting a custom chamber/barrel installed. Ask the gunsmith for a copy of the reamer spec/print at least you have a reference for what was supposed to be put into the barrel.

I don't like renting tools. You don't know how many times the reamer has been used is just one reason.

Later, Frank
Bartlein Barrels

Frank,

So what I hear you telling me is I need to buy the lathe I've been looking at.

I've just recently started buying my own reamers. It certainly makes things a lot easier with regards to knowing what is actually being used on your barrel.