MIT, I seriously doubt the reamer is the problem here. Measure the OD on the freebore section on the reamer, and if that measures OK the reamer won't be the problem. It may be something else, but you very well might be experiencing something I frequently see to one degree or another, especially in canted land barrels. Go in with your long-reach indicator and see if the free-bore section of the throat is smooth. My guess is it's not, which means you have experienced reamer chatter.
It's amazing how much reamer flex can happen in these chambers, and it's especially prone to do this in canted land barrels (it can do it in conventional land barrels also, but not nearly as readily). It will do it no matter how true you have the bore dialed in, how well-fitted the reamer bushing is in the bore, how carefully you feed the reamer in, or how big and expensive your lathe is. Carbide reamers are much stiffer and won't do it as bad, but most reamers are HSS and they will definitely do it!
I started seeing this unexplained reamer chatter phenomenon as soon as I started using canted land barrels quite a few years ago. I called the barrel maker on it a number of times, but he told me I was the only guy in the country he knew of who had seen this. I told him maybe I was the only guy who consistently measures for this all the way through the chambering process, but I see it to this day on about every canted land barrel I chamber. I still use and like canted land barrels, but I just know to be extra careful when chambering them.
I often demonstrate this in my classes (and will be showing this in-depth in an upcoming "Advanced Chambering" DVD) - I'll tell the students exactly when the chatter will start. I will be chambering along and the indicator shows absolutely no chatter whatsoever anywhere in the chamber. When the reamer is far enough in to start cutting the throat, we will then instantly be able to measure chatter through the whole length of the chamber. In every single case, when I can feel the reamer start to cut the throat, I'll tell them we'll now put the indicator in and see what happens - sure enough, we can now measure chatter beginning, not only in the throat area, but in the entire chamber. Usually its very minor and if you didn’t measure for it you’d never know anything was going on, but it’s there and can get worse if you don’t catch it in time.
Then I show them the method I now use to prevent this as much as possible - simply prebore the lands away clear to the end of the neck while you are pre-drilling and boring the body of the chamber. This allows the reamer to be completely seated (and now fully supported) before it finally gets to the throat area as it comes to full depth. When the reamer is fully supported and cutting completely before it finally picks up the throat way at the end, this really minimizes chatter problems.
Now if you have to cut a longer throat with a separate throating reamer after you've done using the main chambering reamer, you really have to be careful the chatter doesn't get away from you, since the body of the throater is completely unsupported. Both the throater and the chamber reamer will benefit by using the wax-paper wrap trick since this dampens the flex somewhat and will help keep you out of trouble.
I've tried quite a few things over the years trying to figure out what causes this and how to prevent it. In my opinion, what happens is the conventional land barrels have straight vertical sides on the lands, so when the straight vertical flutes on the reamer cuts into them, they meet square and there is no side pressure on the reamer flutes so they cut very nicely. But in canted land barrels, the lands are sloped at a pretty good angle, so the flutes of the reamer want to ride up the lands instead of cutting straight into them. Most of the canted land barrels are an odd number like 5 land/groove, so each time a reamer flute tries to ride up a land, it pushes away from that land making the opposing flutes dig into the grooves between the opposite lands (you can usually see this in a borescope if you know what to look for). Measure it and you'll see that the freebore is now 5-sided and not round!
This can happen severely enough that the effective diameter of the freebore section in the throat (the tops of the chatter) is smaller than the reamer itself. The reamer just flexes up and down in these grooves and flexes sideways just enough to form "lands" which is what you're measuring with your indicator. I've often seen it (and measured it) where the reamer chatter in the throat made the effective freebore smaller than the bullet diameter like you've seen here.
Check it out and see if maybe this is what you're experiencing. Hope this helps!
Gordy Gritters
www.ExtremeAccuracyInstitute.com
