Well, I bit the bullet and had a fireforming chamber made for the 30BR from a piece of old barrel. It works great and the cases come out well formed with no groove left on the inside of the neck like the ones necked up with the expander did. I didn't like the idea of turning off the donut created by expanding and leaving the thin spot on the inside of the neck. I couldn't fireform after expanding to flatten out the ridge at the base because the donut was too big to chamber in the .330 neck on my gun, so I decide to fireform from the get-go instead of expanding.
As I was working with the fireformed cases, I discovered another, more traditional type of donut that was caused by the fireforming! After the initial firing to blow out the neck, the case has to be sized to bring the neck back down for turning. I full length sized them and then expanded them to the proper mandrel size with the K&M exapander. Upon turning the necks, I discovered that there was still a hump near the base that the cutter really dug into. It was about six thousandths bigger than the rest of the neck. This hump was NOT in the same place as the one created when initially expanding the necks up from 6 to 30 with an expander. It was right at the base of the neck instead of where the old shoulder of the 6mm was.
This bothered me because the whole idea of making the fireforming barrel was to avoid any inconsistency in neck thickness. What was causing this new irregularity? I measured a freshly fireformed case and the neck was .330 from end to end, perfectly formed. Hmmmmm. I pushed a bullet into the neck and it hit a stop right at the base and wouldn't go any further. I tried several cases with the same result. I sized and expanded it then measured again. Now the hump was there! I grabben another case and full sized it and measured before expanding. The hump was there again! It turns out that fireforming turns part of the old shoulder into new neck area and that area seems to be thicker than the old neck was. The expander ball on the sizing die was pushing the thick part from the inside to the outside of the neck. The turning operation then trims it to the same thickness as the rest of the neck, so I wind up with a uniform neck from top to bottom, but it had me going for a bit! If anyone else goes this route, be forwarned.
By the way, the gun is living up to its reputation. It's the first TRUE 1/4 moa gun I've ever built with most groups better than that. It won the first local match I entered it in with only 75 rounds on the barrel AFTER the match. I used the first load I tried, which was 34.5 grains of H4198 behind a 117 grain Fowler match bullet (a tack driver from the start).
Ron
As I was working with the fireformed cases, I discovered another, more traditional type of donut that was caused by the fireforming! After the initial firing to blow out the neck, the case has to be sized to bring the neck back down for turning. I full length sized them and then expanded them to the proper mandrel size with the K&M exapander. Upon turning the necks, I discovered that there was still a hump near the base that the cutter really dug into. It was about six thousandths bigger than the rest of the neck. This hump was NOT in the same place as the one created when initially expanding the necks up from 6 to 30 with an expander. It was right at the base of the neck instead of where the old shoulder of the 6mm was.
This bothered me because the whole idea of making the fireforming barrel was to avoid any inconsistency in neck thickness. What was causing this new irregularity? I measured a freshly fireformed case and the neck was .330 from end to end, perfectly formed. Hmmmmm. I pushed a bullet into the neck and it hit a stop right at the base and wouldn't go any further. I tried several cases with the same result. I sized and expanded it then measured again. Now the hump was there! I grabben another case and full sized it and measured before expanding. The hump was there again! It turns out that fireforming turns part of the old shoulder into new neck area and that area seems to be thicker than the old neck was. The expander ball on the sizing die was pushing the thick part from the inside to the outside of the neck. The turning operation then trims it to the same thickness as the rest of the neck, so I wind up with a uniform neck from top to bottom, but it had me going for a bit! If anyone else goes this route, be forwarned.
By the way, the gun is living up to its reputation. It's the first TRUE 1/4 moa gun I've ever built with most groups better than that. It won the first local match I entered it in with only 75 rounds on the barrel AFTER the match. I used the first load I tried, which was 34.5 grains of H4198 behind a 117 grain Fowler match bullet (a tack driver from the start).
Ron
