I notice one mail order house sells short chambered barrels in .416 Taylor and .458 Win.
The same house touts barrel installation can be accomplished by hand reaming the chamber to finished depth - no lathe required.
It seems that a good polished chamber finish with at least 400 grit silicon carbide would be a requirement for slick chambering and extraction with dangerous game rounds? A lathe is required to do this properly. What is the conventional wisdom for a good finish in a DGR chamber?

There is nothing wrong with polishing the chamber to some degree, but a mirror finish is not required.

Since probably 99.5% of big bore rifles are never used on dangerous game (feral hogs don't count), a hand-reamed chamber would not be a huge impediment.

George

quote:

Originally posted by GeorgeS:

Since probably 99.5% of big bore rifles are never used on dangerous game (feral hogs don't count), a hand-reamed chamber would not be a huge impediment.

George

Can you post your evidence of that? No theories, just evidence.



Get the idea?

quote:

Originally posted by 500grains:

quote:

Originally posted by GeorgeS:

Since probably 99.5% of big bore rifles are never used on dangerous game (feral hogs don't count), a hand-reamed chamber would not be a huge impediment.

George

Can you post your evidence of that? No theories, just evidence.



Get the idea?

I've not had experience with DG rifles unpolished but have with big game rifle chambers unpolished.....and as long as the reamer is sharp it will cut a 63 RMS or better finish and this works well for extraction.

The only value in polishing as far as I can see is to improve extraction.....or am I missing something?

Vapo,

I was not actually commenting on chamber polishing, just illustrating a point to George that it is not productive to run about the internet on a "Prove it!" jihad.

As for polishing a chamber, my preference is for both a polished chamber and a finely polished match grade bore that looks like a mirror. They certainly foul less.

I'm not certain of this being factual as I'm not an engineer, but I think that an overly polished chamber could result in hight bolt thrust so I don't polish chambers more than 320 grit.

Can you hand ream a chamber? The answer is yes. I have built 5-6 competition M1 Garands that way ( actually turning the reamer from the muzzel). Why does it work? The pilot assures the reamer goes straight. Same applies to a DGR barrel.
All my reamers leave a near perfect finish but you don't want a high finished chamber at the slight roughening of a 400 grit chamber allows the brass case to grab the chamber walls when the cartridge is fired. Thus minimizing bolt thrust. You will see pressure signs much earlier on a highly polished chamber than with a 400 grit chamber.
Personally I use a lathe to chamber short chambered barrels as I often find I need to properly square both the receiver face and barrel shoulder before reaming. I always do the last .003 by hand although supported by the lathe tailstock. Of couse all my guns shoot .5 MOA or better, but then I'm a perfectionist.-Rob

Dan,

I did say 'probably'. That allows a latitude not provided in your declarative, and still unsubstantiated, statements that

quote:

Originally posted by 500grains:
1:16.5 is spec for the 416 but most makers went with a faster twist at 1:14 to improve performance. Performance can be further improved by going with a 1:10 or at least 1:12. (emphasis added- GS)

George

George, that statement is accurate. Do you have info that says it is not? Or are you moderating me to put a 'probably' into it?

quote:

Originally posted by Toomany Tools:
I'm not certain of this being factual as I'm not an engineer, but I think that an overly polished chamber could result in hight bolt thrust so I don't polish chambers more than 320 grit.

Polish of a chamber has almost nothing at all to do with bolt thrust.

The brass case will stretch to provide all the thrust the powder pressure can provide and the difference in bolt thrust from a polished chamber compared to a unpolished chamber can be measured in less than a hundred pounds if at all.

quote:

Originally posted by 500grains:
George, that statement is accurate. Do you have info that says it is not? Or are you moderating me to put a 'probably' into it?

You are being deliberately non-responsive, and quite unnecessarily.

Your statements can either be corroborated or not. If not, then they are conjecture, opinion, rumor, et cetera, not fact, and you should either preface such statements with disclaimers, or be prepared to be challenged on them.

George

quote:

Your statements can either be corroborated or not. If not, then they are conjecture, opinion, rumor, et cetera, not fact, and you should either preface such statements with disclaimers, or be prepared to be challenged on them.

That doesn't hold true for signatures as well does it...or is libel fair game too?

Just as an observation, I once chambered a 30-06 AO and was in a rush so I didn't polish the chamber. Fired the gun with high end loads and thoroughly stuck the bolt. Had to hammer it open. Case head had bright ejector marks on it. Thinking it was because of the chamber, I polished the chamber to 400 grit and fired the same load again. Bolt opened and extraction was normal. Been so ever since and with the same loads.-Rob

quote:

Originally posted by 500grains:
I was not actually commenting on chamber polishing, just illustrating a point to George that it is not productive to run about the internet on a "Prove it!" jihad.

quote:

Originally posted by 500grains:

The following persons are on my ignore list due to their repeatedly posting dishonest and untrue information:

Funny, how can one square these positions?

quote:

Originally posted by WillSmith:
If you don't START nothing, there won't BE nothing

A chamber should have a good even finish. overly polished chambers do lead to less case adhesion to the chamber walls.

as far hand finishing a reamer, you will NOT get as good a finish as POSSIBLE with a lathe.

400grit, with a mechicanl/electrical machine turning it render a far better finish then 400grit on a dowel turned by hand.

remember, some folks actually do this work, and some folks just talk about their orders.

jeffe

quote:

Originally posted by jeffeosso:
A chamber should have a good even finish. overly polished chambers do lead to less case adhesion to the chamber walls.

Educate me a bit here please.....why do we care about case adhesion to chamber walls?

Actually adhesion seems to be a bad thing.
Check out Varmint Al's finite element analysis of the effects of the surface finish on case life and bolt thrust.
Varmint Al's

quote:

Originally posted by ireload2:
Actually adhesion seems to be a bad thing.
Check out Varmint Al's finite element analysis of the effects of the surface finish on case life and bolt thrust.
Varmint Al's

I tried to understand him......but I'll let others read it and tell me what it says.....it's too much analysis for me.

quote:

Originally posted by vapodog:

quote:

Originally posted by ireload2:
Actually adhesion seems to be a bad thing.
Check out Varmint Al's finite element analysis of the effects of the surface finish on case life and bolt thrust.
Varmint Al's

I tried to understand him......but I'll let others read it and tell me what it says.....it's too much analysis for me.

Summary Table with Estimated and Measured Coefficients of Friction
See the Friction Test Results
Estimated/Measured Coefficient of Friction (u) Comments Maximum Plastic Strain
(%) Cartridge Case Wall Radial Thinning (inch) Maximum Bolt Face Load (lbs)
0.55 Very rough chamber, rough reamer finish with tool marks 14.39 0.006054 4435
0.41/0.37 Rough chamber finish, 320 grit finish (200 grit) 13.42 0.006328 4386
0.35/0.29 Smooth chamber, 600 grit 12.87 0.006046 4639
0.25/0.27 Smooth chamber, crocus cloth or smoother 11.10 0.005415 4477
0.19/0.19 Polished chamber Flitz 9.69 0.004858 4565
0.11* Probably not possible: Polished chamber, polished brass with grease 7.92 0.004481 4787
0.01 Friction this low is probably not physically possible and would be dangerous 14.17 0.009328 (case head expansion that could cause the case head to rupturing) 7656
0.00 If Case Head Separation (Hand calculation
f = pressure * area) 8785

* Note: I am not suggesting using lubricant on ammunition brass. I am merely trying to identify how this low of a friction coefficient might possibly be obtained.

Ah, okay...

two issues here, but both are "pii", right?

first, bolt thrust ...
it is tested/measured (or calaculated to assume) using a resized case, strong load, and GREASED to prevent all side wall adhesion possible.

case head sep? ... it's early for me (was up late)... and going through al's stuff (which is amazing) is a little tough.

sure, there's a POINT where gripping the chamber wall can be bad..., but unless grossely exaggerated, that is a non-issue... undersized brass to chamber, big load, high pressure, and you could have a casehead seperation.. sure, could happen... not GOING to happen in a normal finish rifle...


these two items are VERY VERY different. bolt thrust with a clingly chamber wall is thought to be reduced... Heh, I am not making this up... this old codger named PO Ackley seems to have though that up, if you read his works on the case design... straight walls, to him, produced the 30/30 ackley... and it's not just a question of capacity.

case head seperation due to clingy walls, well, there MUST be a headspace issue involved with that, as well, as the analysis shows the cases flowing backwards underpressure and a smooth sided chamber comes back faster.


however, none this is not exactly what I was meaing

If you chamber with a boring bar on a CNC lathe, you have to hit it with 400 or 600 to ROUGH IT UP, as when properly done, there's nil finish marks on the chamber. Mirror finish doesn't begin to describe it. Fritz 454 can tell you all about it.

jeffe

quote:

and going through al's stuff (which is amazing) is a little tough.

Jeffe,
I've had enough calculus and other high level math in my life to understand Big Al's article seems to have merit and I'll give him clearance on this.

I still didn't comprehend his point but it's also not highly relevant as I disagree with the initial premise anyway.....

Any action worthy of a big bore should be able to withstand the bolt thrust regardless of the finish in the chamber.....that point is irrelevant to me. (I still don't buy reduced bolt thrust with rough chambers even if it is irrelevant)

The critical issue with the DG rifle is extraction and I'd be hard pressed to understand an unpolished chamber being preferrable to one that has at least some polishing.

vapo,
maybe its the binary condition of the term "unpolished"

i like a smooth finish in my chambers, and generally run dowel/400gr with kroil on it in them, after I am done cutting them.

this is"finished" to me, not "polished" ... polished has a different surface finsh... "mirror" comes to mind..

well, you cut the chamber and if the reamer is finished well, it should cut a pretty darn good chamber... and a little sanding won't hurt...

but it you cut it, with a reamer, THEN polish it, well, I don't do it that way.. might be just out of habbit.

I agree that both conditions are probably PII, just the way I do it


heh, add the point that I was up till 3 something (tip, don't drink coffee at the office party when everyone else is drinking mixed drinks... at least not one black coffee to every cocktail you boss has !!) ... and yeah, it was kinda hard to go through.

I do buy the boltthrust/case wall thing... why else would the ideal test be a lubed case?

big bores are actually where bolt thrust COMES into play.... larger surface area on the bolt face.... but, on a scale of 0 to 10, the importance of this, to me, is about .001

but, it's like saying "that's a black car" "yep, but it's a blue black " "nah, it's a purple black"...

it's certainly not a pink car


jeffe