Polished brass. Can it be too smooth?

[McHaggis]

Cold lazy Sunday so watched some youtube. Watched a couple of vids showing how a chamber is cut into a new barrel. Both smiths ran fine sandpaper into the chamber area to clean it up after the reamer was done then both “surfaced” the chamber so it wasn’t too smooth. The reason for this is to provide case grip to reduce case thrust on the bolt. Sounds perfectly reasonable and sensible but it got me to thinking about the cases.

To this end can the brass be too smooth?

Just putting it out there to provoke thought but I also have an issue with a light 6.5creed load which I’m chasing.

 

[Dusty Stevens]

In an engine the optimal finish is the crosshatch pattern in the cylinder just like the youtube guy did but the rings are mirror smooth. Once you run your brass thru a die a few times its pretty slick too but your die has the same finish as the chamber. As long as one is rougher than the other its good. Like glass rubbing against glass as opposed to sandpaper- the sandpaper just glides on over it

 

[KenOath]

Cold lazy Sunday so watched some youtube. Watched a couple of vids showing how a chamber is cut into a new barrel. Both smiths ran fine sandpaper into the chamber area to clean it up after the reamer was done then both “surfaced” the chamber so it wasn’t too smooth. The reason for this is to provide case grip to reduce case thrust on the bolt. Sounds perfectly reasonable and sensible but it got me to thinking about the cases.

To this end can the brass be too smooth?

Just putting it out there to provoke thought but I also have an issue with a light 6.5creed load which I’m chasing.

One thing I have experienced is firing brass that has a little water on it from light rain on my box of bullets one FClass match... had fairly heavy bolt lift taking those cases out I’m assuming because they didn’t grip the chamber wall as much as they should have obviously due to the brass being wet... I wouldn’t imagine having highly polished brass would be doing you any favours, although I haven’t tested this before just an assumption...

 

[Ledd Slinger]

Tag

 

[dmoran]

Other then "eye candy" I see no gains or advantages to polished brass. Judging by the amount of smoke on the necks/shoulder, I'm convinced that polished brass does not seal as soon or as consistently. Myself use micro-fiber clothes to wipe them off and brushes for the internal, not ever media tumbling them. I do ultra-sonic clean them once a year or so, which totally cleans the cases internally, which is my primary concern.

 

[dcali]

I’ve heard of chambers being too smooth, but not brass. But I don’t see why brass couldn’t be too smooth. I think you’d have to polish it purposefully to get it there, though. But with anything to do with friction, speculation is pointless you just have to test it.

 

[dcali]

Other then "eye candy" I see no gains or advantages to polished brass. Judging by the amount of smoke on the necks/shoulder, I'm convinced that polished brass does not seal as soon or as consistently. Myself use micro-fiber clothes to whip them off and brushes for the internal, not ever media tumbling them. I do ultra-sonic clean them once a year or so, which totally cleans the cases internally, which is my primary concern.

I’ve found no reason to clean brass other than to keep my hands clean. I still clean it, but not for accuracy.

 

[dkhunt14]

One thing I have experienced is firing brass that has a little water on it from light rain on my box of bullets one FClass match... had fairly heavy bolt lift taking those cases out I’m assuming because they didn’t grip the chamber wall as much as they should have obviously due to the brass being wet... I wouldn’t imagine having highly polished brass would be doing you any favours, although I haven’t tested this before just an assumption...

A couple of years ago at a match, it was raining a fine misty rain. The wind was blowing toward the firing line and my shells were sitting in a loading block. The sighters went ok and the farther down the record string I got, the more stiff bolt lift I had. I know the cases got wetter the longer they sat there. By the last record round, I could hardly open the bolt. I figured the water took up room in the chamber and hydraulically built pressure. Those cases were ruined and the expansion on the case heads were great. It also happened to another shooter and we talked about it. His cases were also over expanded and he also got the hard sticky bolt. Matt

 

[Laurie]

In an engine the optimal finish is the crosshatch pattern in the cylinder just like the youtube guy did but the rings are mirror smooth. Once you run your brass thru a die a few times its pretty slick too but your die has the same finish as the chamber. As long as one is rougher than the other its good. Like glass rubbing against glass as opposed to sandpaper- the sandpaper just glides on over it

Good explanation. I've occasionally wondered about this issue too, although it's not exactly the sort of thing that keeps me awake at night.

The rest of you - shame on you. Everybody knows shiny ammo shoots better.

Rushty - there are likely several things happening with wet ammunition during a match. (We know a lot about the results in the UK, because 1) we shoot a lot in rain often with winds blowing drops into loading ports / chambers, onto scope ocular lens etc and 2) an older generation which cut its teeth with the Lee-Enfield designs both in 303 and adapted for 7.62/308 discovered the adverse effects of water in the action and chamber - the rifles are actually only marginally safe in really wet conditions, very strange indeed considering the British Empire fought with L-Es all round the world and in many places that are even wetter than Salisbury Plain, the traditional home of British Army training.)

One factor is probably water film between the case and chamber. Likely worse though is that serially chambering wet ammunition progressively moves water through the chamber into the neck area of the chamber, leade, throat and rear end of the rifled barrel where the resulting reduced clearances massively increase pressures as water is virtually incompressible. It's when it gets to the front of the chamber / throat that pressure issues get out of hand. I was plotting for a GB FTR team in the F-Class European Championships at Bisley many Novembers ago when the team matches had to be abandoned thanks to very heavy rain allied to 20 mph plus winds from around 5 o'clock perfectly aligned to put water through ports on right-hand actions. One could see the problems appear then worsen as the first pairs of shooters started on the first match - around score shots 4 or 5, the first F-Open problems appeared with the occasional blown primer, by half way through nearly all F/O were in difficulty with many competitors starting to give up of their own accord, and FTR shooters were now starting to see hard bolt lift and the first leaking and blown primers. Using a chamber cleaning stick and mop wrapped in paper kitchen toweling got chambers dry but wouldn't cure the problems we found. (The team matches were then formally abandoned.) What British TR / Palma and F shooters have learned over the years is to make every effort to keep the ammunition dry on its short trip from ammo box to chamber as that's the primary means of water transfer.

A final thought on the issue to toss into the pond. The British military proofed rifles for decades using proof cartridges with oil externally applied to the case walls. This reduced the case to chamber grip and thereby increased the case-head back-thrust onto the bolt. Standing up to this was what mattered and where pressure was recorded. Older British proof on Lee-Enfields and suchlike has the proof house stamp plus a figure in tons, 20 tons for 7.62 Enfields IIRC and this refers to case to bolt thrust. It is interesting that while the practice achieved this end, the resulting stresses were still well within the limits of what the weak L-E action would safely accept. Even if a polished case reduces grip a little, it'll come nowhere near doing so as much as a film of light mineral oil.

 

[Laurie]

A couple of years ago at a match, it was raining a fine misty rain. The wind was blowing toward the firing line and my shells were sitting in a loading block.

You'd never see that in a UK match Matt. If there is any chance of rain at all, ammo is kept in closed boxes and the boxes placed under cover usually in competitors' 'Weather Writers' with all sorts of alfresco 'rifle umbrellas' (clear heavy-duty polythene sheeting usually attached via the scope elevation turret) protecting both the action and vitally the cartridge transfer between box and port. Although unpleasant shooting in these conditions, a good set-up works well in keeping water transfer and pressure increases down to manageable levels. As wind often accompanies the rain, the protective sheet being blown off the rifle and downrange is a hazard though if it isn't really well secured.

 

[Bart B.]

When I did tests on 308 Win case shoulder setback from firing pin impact, nickel plated cases' shoulders set back .006" to .007", over twice as much as regular brass ones. 28 pound striker spring, 2.5 ounce pin striking primers at near 20 fps.

The Federal cartridge rep at the Nationals I discussed this with knew this was common. And more bolt thrust with them was normal. The reasons they were discontinued. But they sure fed easier from box magazines in rapid fire matches.

 

[Laurie]

When I did tests on 308 Win case shoulder setback from firing pin impact, nickel plated cases' shoulders set back .006" to .007", over twice as much as regular brass ones. 28 pound striker spring, 2.5 ounce pin striking primers at near 20 fps.

Very interesting Bart. I'd not heard that before - yet another good reason to avoid nickel plated brass like the plague!

 

[Bart B.]

In an engine the optimal finish is the crosshatch pattern in the cylinder just like the youtube guy did but the rings are mirror smooth.

Auto engines are that way to keep a thin oil film between cylinder wall and piston rings. They have to last a dozen hours to thousands of hours in cars depending on use.

Barrel bore surfaces are typically a 10 to 15 microinch finish length wise in the best match ones. The have to last only a few seconds of use. Such surfaces enable least bullet jacket fouling.

 

[Dusty Stevens]

Auto engines are that way to keep a thin oil film between cylinder wall and piston rings. They have to last a dozen hours to thousands of hours in cars depending on use.

Barrel bore surfaces are typically a 10 to 15 microinch finish length wise in the best match ones. The have to last only a few seconds of use. Such surfaces enable least bullet jacket fouling.

Chambers and barrel bores are 2 totally different things

 

[Bart B.]

Chambers and barrel bores are 2 totally different things

I agree, but opinions range full spectrum on how smooth chambers should be.

 

[Dusty Stevens]

The general convention among accuracy riflesmiths is a 320 or so grit crosshatch pattern. Of course opinions vary wildly outside of that group but it works well with cases that have extreme pressure by most standards

 

[Bart B.]

The general convention among accuracy riflesmiths is a 320 or so grit crosshatch pattern. Of course opinions vary wildly outside of that group but it works well with cases that have extreme pressure by most standards

I've heard that crosshatch method is good. But how much peak pressure is extreme for a cartridge SAAMI spec'd at 65,000 psi?

 

[Dusty Stevens]

We dont use saami spec cartridges. 80k is not out of the ordinary these days and we dont have case extraction issues (most people)

 

[Bart B.]

80k is not out of the ordinary

Proof loads. Plain old proof loads.

Hope some readers don't feel ok shooting their stuff at that level.

 

[LA50SHOOTER]

In regards to the OP's question - Short answer IMO is No, it can't be to smooth. Unless someone deliberately polished it to a level of finish where it was like glass & even then, I don't know. - What is important is it is clean, dry & free of any oil or lubes.
- I don't tumble brass for accuracy type rifles, I wipe it with a micro-fiber cleaning cloth (as Donovan stated he does). At the neck if it has carbon residue I give it a quick spin with a 0000 artificial steel wool pad & wipe it with the micro-fiber cleaning cloth. - Latter in the brass' life when its had a good number of firings I'll assess if I should clean it with stainless steel pins in a tumbler or just replace it with new. - We already spend a lot of time preparing our brass which I feel is a good thing & worth the effort, but polishing it to a mirror finish isn't really necessary IMO.

My .02