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I always hear people talking about " tight custom chambers" and "sloppy factory chambers.". Presumably, because factory reamers are used on more chambers and wear down. But you'd think the opposite would be true, i.e., the more the reamer wears, the smaller (tighter) the chamber would be. So what is the story with tight custom chambers?



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Not a writer but factory chambers have to work with ALL brands of factory loaded cartridges, custom chambers are designed to have ammo loaded just for that one chamber so tolerances can be much closer.


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smokepole,

Another difference is that custom reamers are often made of harder steel, so they don't wear as quickly. On of the gunsmiths I know had chambered a few hundred .300 Winchester Magnums with the same high-quality reamer, and hasn't seen any measureable wear. As a result a custom reamer can be made to cut a SAAMI minimum chamber.

Factory reamers tend to wear more (or at least those used in many factories) so they're typically made to maximum specifications, then used only until they cut a minimum SAAMI chamber. This is one of several reasons factory rifles can vary more in accuracy.


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Thanks John.



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"So what is the story with tight custom Chambers"?

When I order a reamer from PT&G, I have them send me a print of a standard SAAMI specifications in that cartridge and another print of Minimum SAAMI spec Match reamer.

I will then take the match reamer print and make changes on dimensions that I deem necessary. For instance, if you look at the dimensions of the print, compare that to the brand of brass that you want to shoot, you may decide to make changes to eliminate the bulge at the base of the case. The Standard SAAMI print MAY indicate of what you can expect from dies and brass, used as a guide only.

Or, you may see that the print indicates that the neck is expanding .007 on the brand of brass you intend to use, which may lead to early case failure(splitting) and you want to change that to a tighter dimension where the brass only expands.002,.003 or .004.

As you measure your favorite brand of brass, and you compare to the print, you may see the potential for massive expansion in the Webb. So, you alter the dimension in the webb so that the brand of brass will better fit the Reamer specs.

In as far as factory chamber dimensions, they are all over the map...and that is an understatement. Also, some European companies have standards that are typical for Europe and American standards may be different(freebore).

Smokepole, when you hear the term of "tight custom chambers", someone might be explaining how he or the gunsmith altered the reamer that they ordered, also it might be just a minimum spec match reamer for that caliber. In addition, the person that mentioned, "tight custom reamer" may be dealing with their first custom rifle that does not have a sloppy chamber, just a std SAAMI chamber.

Once you have worked with a rifle with minimum SAAMI spec chamber installed by a gunsmith that has properly aligned the chamber with the bore, you are spoiled for life(reloaders only).

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Yeah, the factory dimensions can be all over the place, especially in older rounds, and particularly European cartridges. My 6.5x55 is chambered with a minimal PT&G reamer, and so far NO American brass will enter the chamber unless the rear end is sized down, but Lapua brass is perfect. Limiting myself to Lapua brass is no biggie.

Have also owned an FN commercial Mauser that had such a short throat some American "enhanced velocity" factory loads blew primers--and got over 3000 fps from the 22" barrel. Have also seen custom chambers for Weatherby cartridges that blew primers on the first shot with factory ammo.

All sorts of stuff happens if we vary chamber and bore dimensions.


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In many cases it may be the factory methodology as much as the tooling dimensions which result in larger chambers. In other words, they may not take the care in alignment that a gunsmith must. Remington, for instance, finishes their chambers with a reamer which is run through the bore and clamped in a collet. The reamer turns while the barreled action is held in a vee block clamp. The bolt is closed on the base of the reamer and when the bolt closes, the chamber is done. If the bore is curved (common), side pressure may cause the reamer to cut a chamber which is misaligned or over sized.
Occasionally, the reamed finish is so poor that excessive polishing is required to get the rifle to function at all. In many cases though, the factory rifle is simply not built to SAAMI specs at all. This is the only explanation for the wide variation in throat length, diameter. or configuration.
When it omes to set-up errors, it is not only the factories which make them. I have seen a lot of custom chambers which were over sized, eccentric, or over polished. One gunsmith I know was complaining that his reamer was cutting over sized chambers and he was going to send it back. The reamer measured OK and it turned out the problem was in his lathe set-up in combination with poor technique.
I am not immune. I once had offset my tailstock slightly (about ten thou)to taper a shaft and din't re-align it right away. I forgot all about it and was chambering a 30/06 barrel a couple days later. The reamer didn't seem to be cutting all that well and when I stopped to check chamber depth, it was obvious the chamber was oversized at the base. I realized immediately what I had done but it was too late and I had to buy a nother barrel for that customer. I still have that barrel under the bench as a reminder.
Special reamers, as ordered by gunsmiths, are usually special only in throat dimension although some will order reamers which are very tight to minimize brass expansion at the web or at the neck. In general such alterations in SAAMI specs are important only in the mind of the guy doing the ordering and most will find the SAAMI minimum will work just fine.
I think the intended use of the rifle is the one factor which may influence chamber dimensions. If one is making a custom hunting rifle, that rifle is supposed to work better than the run-of -the-mill factory offering. If the chamber is too tight and the rifle is fussy to use and load for, it's not better.
On the other hand, if a rifle is built as a precision varminter or amatch rifle, the gunsmith may vary chamber dimensions in an attempt to improve performance at the target. A benchrest rifle is expected to require considerable brass preparation, for instance.
Often times, a customer may want to load a given bullet to a specific overall length. In this case a throat may have to be cut longer than standard. The gunsmith will either order a reamer with a special throat or he may elect to cut the throat with a separate throating reamer.
Although I am convinced that reamer dimensions have little effect on rifle performance at the target, I have been as OCD as anyone at times when agonizing over the spec sheet so I understand how it can go.
To sum up, it is true that "tight custom chambers" exist but they are not always a good thing. Concentricity and alignment means a lot more than dimensional variation. GD

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Thanks GD, very informative.



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greydog,

I have seen the results of some of the problems you mention through my borescope more than once.

A number of years ago a friend had a Remington 700 Classic in .250 Savage that shot extremely well. A gunsmith friend told him it would work much better if rechambered to .250 AI, and guess what? The gunsmith had a .250 AI reamer.

Eventually my friend caved and let the gunsmith do the job, which included setting the barrel back a couple threads to totally clean up the old chamber. The rifle never shot worth a darn again. I would guess the factory chamber job was well-done, because the rifle shot so well, but do know the rechamber wasn't, because I got to look at it through my bore-scope: The rifling was obviously much longer on one side of the throat than the other. (I reported this off-center rechamber job in an article years ago, and the magazine got a nasty letter from a reader, saying it was impossible for a chamber to be off-center, due to the pilot on the reamer.)

But have also seen the same thing, a number of times, in the throats of various factory rifles. The really interesting part is sometimes they still shoot pretty well!


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I agree with GD.

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After reading Mule Deer's last post it reminds me why I nit pick so much about who is going to fit and chamber barrels. I listened for over a year before a recommendation came to me about a guy who did lots of work for benchrest shooters and was fast in getting his work out.

My son is a machinist and owns his own shop. He told me how important it was to use someone who has a very good reputation and knows how to set up the lathes to counter vibration. Needless to say I mailed two barrels to a fellow on the east coast and had the work back in 30 days. Both guns are 1/4 minute or less in accuracy. I might have spent a little more than what the average gunsmith charges, but I got an excellent job done by the man. The only time of my life I could consider myself as an "elitist", is when it comes to rifle accuracy.

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Having an idea of how the reamer works can allow one to correct a misaligned chamber to a certain extent. The things is, even a close fitting pilot will have SOME clearance (usually on the order of .0001" radially or so) and there is also some clearance between the floating pilot and the reamer (another tenth, perhaps). On top of this, there is the potential for a bit of flex. When approximately 3/4 inch of the reamer body is cutting, the forces on the body can over ride the pilot. The body of the reamer tends to cut to the center of the existing hole so if one wants the throat of the chamber to be concentric, the reamer body has to have a start which is concentric to the bore at the throat.
In the case of the 250 AI to which Mule Deer alluded, the original chamber may well have been concentric at the throat but may have been misaligned so that the base was not perfectly concentric with the throat. When the gunsmith rechambered with the AI reamer, the body of the reamer was influenced by the body of the chamber and the pilot may not have even been in contact yet. With an eccentric start, the reamer produced an eccentric throat.
So, how could he have avoided this? The best way would have been to indicate the throat of the original chamber so that it ran at "zero". Then, if the chamber was running eccentric at the base (and it probably was), he should have used a boring bar to true up the chamber to within about .005" of the diameter of the AI reamer at the shoulder. It would not even have been necessary to bore the whole body; an inch or so would have been more than sufficient. Now, with a good, concentric start, the reamer would have cut a good, concentric throat.
Of course, one has to be a little careful with this boring bar stuff too. On another memorable job I miscalculated the angle and roughed out perfect 308 Norma chamber. Perfect that is except for a ring up near the shoulder where I had bored a little too big and left a groove about .002" deep. This is another barrel which resides under the bench and one which cost me a new barrel (it's a wonder I ever made any money!). I never even noticed the flaw until I tested the rifle.
As MD mentioned, an eccentric chamber can shoot amazingly well. About thirty years ago, I had occasion to fit a very crooked Bauska barrel to a mauser action. This barrel was more than a little curved and I, quite frankly, did not have the knowledge to deal with it at the time. I did the best I could but the throat was visibly eccentric. Not by much; probably about a half thou. Still, I didn't think much of it and told the owner so. The thing was, that rifle (a 6.5/06) shot! He showed me a bunch of two hundred yard five shot groups which hovered around an inch and a quarter. Not half bad for a lightweight sporter. So you never know. GD

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GD,

My custom 6.5x55 is very accurate, as it should be with a minimum chamber in a Lilja barrel, installed by well-known accuracy gunsmith. But it isn't any more accurate than a 6.5x55 Ruger 77 Mark II I once owned, and like a dumb-ass rifle loony sold so I could buy something else. That was before I had a bore-scope, and I wonder what the chamber of the Ruger looked like. It might have been all screwed up, or practically perfect!


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Sometimes, that borescope just tells us things we would be better off not knowing! I bought a Ruger in 7x57 a while back. I didn't like the long throat and the chamber was a bit crooked so I set it back and fixed it up. It shot great and all was well in the world. When cleaning it one day, I decided to have a look with the 'scope. Under magnification, that barrel just was not attractive and I just couldn't handle it. I re barreled it with a much better looking barrel and did a hell of a job on it, if I do say so, but it has never shot as well as it did before and it is only through the excercize of considerable willpower that I resist taking it out in the yard and throwing rocks at it. GD

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Ha! I remember how disappointed I was years ago, when my new bore-scope didn't reveal all the secrets of the rifle universe.

Probably the best use I've gotten out of mine is seeing how well various bore-cleaning products work. But even that doesn't predict how well a barrel will shoot. Some of them love being dirty....


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Originally Posted by Mule Deer
greydog,

I have seen the results of some of the problems you mention through my borescope more than once.



Eventually my friend caved and let the gunsmith do the job, which included setting the barrel back a couple threads to totally clean up the old chamber. The rifle never shot worth a darn again. I would guess the factory chamber job was well-done, because the rifle shot so well, but do know the rechamber wasn't, because I got to look at it through my bore-scope: The rifling was obviously much longer on one side of the throat than the other. (I reported this off-center rechamber job in an article years ago, and the magazine got a nasty letter from a reader, saying it was impossible for a chamber to be off-center, due to the pilot on the reamer.)

But have also seen the same thing, a number of times, in the throats of various factory rifles. The really interesting part is sometimes they still shoot pretty well!


John,

I have also witnessed the off-center chambering on at least four barrels, as seen through my borescope. I never knew how it happened, but rifling would start on one side of the barrel way before it would be evident on the opposite side.

On one new Remington barrel, it caused Remington factory .308 ammo to give high pressure signs immediately. As you noted, the gun shot acceptably well in spite of it. A new custom barrel took care of the problem. Don't know if Remington would have done anything about it.

Borescopes are fascinating devices. Sometimes they reveal some sloppy problems. And sometimes they ruin perfectly good rifles!

WyoM70


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