TomG
posted this
27 March 2013
Like Pat, I've had some barrels that were quite rough looking inside using my Hawkeye borescope. One, a Marlin barrel had crosswise striations from the reaming operation on top of the lands that looked like railroad cross ties. This shot very well and was consistent. I also had a Savage SS barrel that was pretty rough looking and threw fliers in every group. It had two problems. One was a crooked chamber. This was corrected and it still threw the occasional flier. I then found a defect in the top of a land 3/4 ” inside the muzzle. I cut it back to good rifling and re crowned and it shot varmint bullets in the low 4's. It was still rough but it shot extremely well.
Bottom line: The surface finish has much less to do with the accuracy of a barrel when compared to a uniform bore diameter. Polishing didn't help as long as it allowed the tight spots to remain in the barrel. It might make it clean easier but didn't help accuracy. I once polished a barrel interior to 1000 grit finish. It looked beautiful but accuracy didn't change.
I routinely hand lap my match barrels using Clover silicon carbide compound. I start with 180 or 220 grit to remove tight spots and when it feels uniform from one end to the other, I finish with 320 grit or 400 if my memory is good.
MY theory is that the grit breaks down as it's used and the surface finish ends up slightly better than the size of the grit. I've used this method for the cast bullet barrels with good results. I feel the small valleys in the surface of the barrel provide a place for the lube to reside from shot to shot. Thus the barrel can become seasoned. I've experimented at length as to how much lube is needed. With my #24 lube, I found that .4 grs. weight of lube on a 30 cal. bullets was best and if too much lube was used it would cause lube purging fliers. It wouldn't surprise me to see the lube purging made worse by a very smooth surface barrel. At the time I did the 1000 grit lap job, I only looked at accuracy with light loads in a 308 with cast bullets. At that time I didn't test that gun for the optimum lube on the bullets.
Chrome Moly vs. 416 R stainless? It doesn't make much difference as long as the bore variation is held to one to two tenths TIR from end to end.
I also had a few match grade SS barrels make up with different ratio of lands and grooves. I tested from 75/25, 60/40 and 50/50 ratio. They were all custom made by a well known firm in Rapid City.
One of the barrels had pretty prominent reaming marks remaining on the lands. It was hand lapped and uniform. This barrel was accurate but it would take up to 10 shots for the velocity to stabilize. A smooth barrel of 300 grit finish took only one or two fouling shots. Again, a rough barrel can be very accurate but may take longer to come up to the normal internal friction in the barrel. I was driving 30 cal. bullets over 2500 fps. in a 30 br with Varget and some years shooting 5000 rounds a year in matches and for testing different things. In this case, the company had lost their employee who turned out really good barrels. His replacement made the rough one. I had occasion to visit the plant and meet the QC people They explained that they had lured the old employee with the great skill back and he made me up a new barrel and it shot well. He was a master at running one of the old Pratt and Whitney barrel rifling machines.
If someone has some quantified accuracy data on polishing barrels for accuracy, please let us know. My opinion from some limited experience is that it's not worth the effort and can even be detrimental to performance with cast bullets.
I once questioned a match barrel maker from Montana who had a lot of record breaking and match winning cast bullet barrels out there. I was shooting his barrels in matches and installing a lot of them for my customers at the time. He advised that he felt that a finish lap with 320 grip was best for cast or jacketed bullets. His theory was that if the barrel was too smooth, it would allow too much bullet surface in contact with too much barrel surface and cause undue friction in the barrel. This would lead to bullet metal transfer to the barrel surface.
He felt that the ideal situation for accuracy was having the microscopic peaks in the barrel surface in contact with the bullet surface. This riding on top of the peaks reduced friction and shot better over time. This guy really knows his stuff and had mucho experience. He cautioned against getting the barrel too smooth.
Tom Gray