The problem is that in order to make sure the entire barrel meets specs you going to need a specialist and he will not come cheap!! I am not even sure that the job is worth trying. Your best bet is to get the bore reamed to 52 cal and go from there. It is a very viable caliber that does more than the 50 and almost as much as a 54. The quick and easy way is to buy a new rifle, or barrel. Since your muzzle is already at .501 you should forget about enlarging it.
By the way, I wanted to tell you that Green Mountain barrels are manufactured to within ten thousandth of an inch tolerance, so you are showing some pretty sloopy work in your barrel when it is one thousandth over on the muzzle and 5 thousandths under on the breech end. Also wanted to ask what size is the target you showed us?? If you are holding a 3 inch group or so at 100 yds you can use the rifle as is and get by for deer hunting.

Well, I'm happy to say that I have found a combination that works with the Barnes bullets. It has given me an inch and a half repeatable, even in fairly strong winds. It only works because of the "tight" barrel. I used up the 209 powder with the Shock Wave bullets. In 12 shots I had 3 that were absolutely perfect dead center. But I had the rest all over the place, some 6 inches off. I didn't clean between any shots. All bullets loaded medium to medium hard. They all seemed to seat well. I had 2 times where the primer went off but the powder didn't. For that one container of 209, I had that happen 3 times.

I had some 777 left and some Barnes bullets. So I cleaned the barrel, fired 2 primers, then used one patch with the "barrel butter" using one side only. The bullet just barely loads, especially with the starter, but it seated evry time I used it. The first bullet, which I always consider a throw away, was right about 6 inches. I used saliva only to moisen one patch and used it on one side only. With the barrel butter, the crud ring was hardly noticeable. Then two dry patches on one side only. Then the one barrel butter patch. From dead center, my first real shot was about 2 1/2 left and 2 low. I used exactly the same procedure 2 more times, and no bullet was more than a half inch away from any of the other two. I'm satisfied with that for deer season. It's repeatable. If I use the Shock Wave e-z-load rounds, I just won't use the barrel butter.

Even with the barrel butter, the Barnes loads hard, especially the first inch to about 3 inches. After that, it's just firm, with a good seat against the powder. The bullets were firm to load, but never hard enough where i had to use ALL of my strength to get them to seat, And again, I must say that the barrel butter sure makes the crud ring not that big of a deal. Nothing like before, when it was all I could do, to get the wet patch to even penetrate the ring. So that is a plus too.

These Barnes bullets use the yellow sabots.

Sounds like you found something your rifle likes and you are off to the hunt!!!!! Good for you, I am glad to hear it!!!!!

When I was testing yesterday, we tried running a chronograph on my loads, but it didn't work. It always showed "ERR". Although it worked on a shotgun, I shoot through a tube. It's how the range is set up to reduce noise. It's 6 yards long. It's actually a culvert about 2 feet in diameter. There are 5 tubes, each weith a bench, then a space, then 5 more low benches and 3 high ones (for shooting standing up) on a shorter couse. Each tube has a flat deflector angeling down from the top so you can't shoot over the burm at the other end.

The shotgun guy was shooting in the space between the ranges and not using the tubes. Mr rifle is fairly loud and I didn't want to cause noise complaints.

Are there any made especially for black powder? He said his was about $80. Is that a 'cheap' one?

You might want to checkout the Savage 10MLII. Its a 50, but it can also use smokeless powder, which means less cleanup especially if you hunt several days in a row. My 10ML is setup with a 2x7 scope.
My current deer load is a Hornady sabot with a 240gr HP XTP Mag bullet, that is designed to be use in the 454 Casull cartridge, over a charge of 43gr. of NV110 powder. This load shoots point blank, + or _ 3 in., to 200yd.
My load for Big deer or bear is a Hornady sabot with a 300gr HP XTP Mag bullet, again designed for the 454 Casull, of a charge of 44.4gr of NV100 powder. Again, this load shoots point blank to 200yd. This load exceeds a lever action 45-70 load and can be a thumper!
Both of these loads shoot 1 1/2 inch or less groups at 100yd. I've had much better performance with the XTP bullets than with the pointed SST bullets and the ballistics are very similar out to 200yd.

Thanks SilvertipGrizz, but in this part of Iowa we can only use bow, muzzle loader, or shotgun... only.

I have a Pro Chrono chronograph that I have used for years on everything from archery to high power and it works just fine. I think $100 is reasonable for a aprice. By the way, BE CAREFUL with that Savage 10ML there are reports of them exploding and causing serious injury!! Toby Bridges of NAMHA has documented at least one incident on his website with pictures. www.hpmuzzleloading.com/

Thanks for the warning. I'm going to stick "with the girl I brought to the dance". I wonder why it wouldn't work with mine. Took plenty of time to set it up. Was the right height, aligned just right. It was on a sturdy tripod and I was shooting off a bench. Now I wish I had tried it outside the "tube" to see if that was the trouble. The tube really channels the noise and smoke and concentrates it right on the chronograph. Maybe it needs to be farther out.

I'll look the one you mentioned up and ask them.

I'm assuming the results (if I get one) will tell me if my bullet is too fast or too slow. Is it also a trial and error to figure that out too? Or is there data out there already.

On a fall day, in Iowa, the speed of sound is about 1,100 feet per second. You know... 5 seconds (actually 4.8) for every mile between lightning and thunder. So why don't we hear a sonic boom when a bullet goes by? Aren't they doing 1600 - 2000 feet per second? By the way, at Denver, on a hot summer day, the speed of sound is about 100 mph slower.

Anyway, what speed do you try for?

There is a boom and there is a crack you can tell when stuff starts to get fast. I think you are on to something with putting it farther out when I develop smokeless loads I chrony stuff 10 -12 feet out, I've never bothered with blackpowder I always just use what works. The reason I chrony loads is to predict pressures and estimate trajectories at longer distances most of my blackpowder hunting is under 100 yrds.

Iowa, you do hear a crack when a supersonic bullet goes by. What's scary is hearing the crack then the muzzle blast, meaning that the bullet was coming your way!

You should set up about 12 feet or so from your muzzle. Cloudy days work best, out in the open. Depending on your barrel lenghth and powder charge you should be able to get 2000fps at the muzzle with 777 or Blackhorn 209. You can find load data on the net. The url I listed has tons of hunting and shooting info and if you get to talk to Toby be sure and tell him I ( John from Decorah)sent you!!!
Bullets do break the speed of sound and if you ever work the pits on a rifle range you will hear them crack over head as they go by!!

actually with blackpowder you could do this safely (as long as you are behind the gun. start with a 30 grain charge and work up at the subsonic speeds there will be a boom with no crack. After you get past the 1100 fps threshold you'll hear the boom with the crack added. might not be worth the trouble but it could be fun if you like playing with stuff and wasting powder and lead.

I think I've mentioned this elsewhere but...

When reading about the American Civil War I saw something about the "booming of the rifles and the crack of the muskets". The most common Civil War rifles (US Springfields and British Enfields) had muzzle velocities of about 950 ft/sec while the smoothbore muskets were around 1200 ft/sec.

These are my sabots that I collected today. The ground is covered with spent shells, so I can't find them all. I noted that my e-z load sabots load the same as the Barnes sabots, but I have to use "barrel butter" to get the Barnes' to load. I tried one today dry and it wouldn't load a full inch.

Can anyone tell me what is going on?

These are my Shock Wave e-z load sabots.