5 lobed chamber

[lrgoodger]

I got my Sinclair concentricity gauge today and all I can say is WOW! It showed me how woefully inadequate my home-made gauge is and the mistakes I made in its construction. It worked well enough, but would only do loaded rounds and not cases, but it was only showing me a fraction of the real runout because I had the gauge tip to close to the end of the bullet.

It showed me how bad the expanded 284 cases really are. Runout on the loaded, unfired cases is 10 - 15 thou. I knew all that downward pressure during expanding was making the necks crooked. I'm glad I had a fireforming chamber made for my 30BR and I'll probably wind up doing one for the 284 shehane now. I'm not totally convinced that firing those crooked cases is going to straighten them out as much as I would like.

Now, I found something more interesting. I noticed that the fired 284 cases were bouncing the needle up and down a half thou with a regular rhythm as I rotated it. I marked a few of the cases and counted the pulses. There are exactly five per rotation on every fired case, both on the case body and on the neck. That tells me my chamber must look like this (exaggerated, of course)!

What could cause this? I know that if the reamer was off center in the lathe, it would just make the chamber oversize near the head. This looks more like the barrel was hitting a bump every 72 degrees as it turned. Maybe the lathe bearing is getting sloppy? I measured fired cases from all the other barrels chambered on this gunsmith's lathe and they are fine (he's done 8 barrels for me).

 

[gunsandgunsmithing]

Just a guess here...Are you using a 5C/R type barrel...probably button rifled?

 

[Erik Cortina]

Maybe reamer chatter or gunsmith stopped the lathe with reamer engaged and withdrew reamer cutting grooves with every flute?

 

[lrgoodger]

Just a guess here...Are you using a 5C/R type barrel...probably button rifled?

It is a Bartlein 5R, but their barrels are cut rifled. I can see what you are thinking, but there is no way even a button rifled barrel could induce enough stress to have this effect clear out on the main body chamber walls.

 

[lrgoodger]

Maybe reamer chatter or gunsmith stopped the lathe with reamer engaged and withdrew reamer cutting grooves with every flute?

I was thinking reamer chatter at first, but that would just result in a larger chamber. I can't see how it could chatter precisely every 72 degrees. The reamer has six cutters.

 

[gunsandgunsmithing]

Just a guess here...Are you using a 5C/R type barrel...probably button rifled?

It is a Bartlein 5R, but their barrels are cut rifled. I can see what you are thinking, but there is no way even a button rifled barrel could induce enough stress to have this effect clear out on the main body chamber walls.

I agree. I missed where you said it was on the body too, till I went back and read it again...interesting.

 

[gunsandgunsmithing]

I use several 5 groove barrels and have never seen any indication of this with my setup, checking chamber runout with an indicator in the chamber while chucked in the lathe. This problem makes me think about the 3 fluted bits that drill square holes. Is it possible that something in the setup caused this?

 

[STS]

Reamer chatter. I had a reamer try and chatter on me that was so subtle that I couldn't feel it in the reamer as it was cutting but I could see it in that surface of the chamber and easily see it with a .0005 test indicator. I corrected the problem in time to save the chamber. It was a 4 flute .17 caliber reamer and that chamber had probably 10 "flutes" in it.

 

[Webster]

It's been a long time since I had the info on cylinders and bores being out of round in a pattern. Usually you need instruments that measure in 0.00001" to detect the pattern. At a smaller level than what you are experiencing. If I remember correctly it's caused by clearances and harmonic vibration in the cutting machine. It has been determined that several distinct patterns in dimensions are created like clover leaf shape of run out. It's scary that you see it at a larger level. I will try to find an article on internet about it and post it.

 

[jonbearman]

I would make a chamber cast to get a real clear picture of what you will tell your smith.This is good knowledge indeed,never heard of this problem before.

 

[lrgoodger]

It's been a long time since I had the info on cylinders and bores being out of round in a pattern. Usually you need instruments that measure in 0.00001" to detect the pattern. At a smaller level than what you are experiencing. If I remember correctly it's caused by clearances and harmonic vibration in the cutting machine. It has been determined that several distinct patterns in dimensions are created like clover leaf shape of run out. It's scary that you see it at a larger level. I will try to find an article on internet about it and post it.

I suppose a harmonic chatter could cause lobes. There was other evidence that it was chatter, too, like the concentric rings running up the leade to the lands right after I got it back from the smith and the fact that the fired brass neck measured 1/2 thou over the reamer neck size. Springback is supposed to make it smaller. I had already sent the smith a borescope photo of the chatter marks on the leade and told him it would probalby be coming back, but this additional evidence pretty much confirms it.

Thanks for the input, guys.