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View Full Version : XCB project. Throat results: 2000+ rounds of HV cast lead = what?



MBTcustom
06-01-2015, 10:40 PM
It has been a question that has been tossed around quite a bit: What happens when you shoot a LOT of HV cast bullets at 2500+ FPS?

After all, this is the recipe to quickly turn your barrel into a crispy critter:
1. Slow powders
2. Overbore cartridge
3. High operating pressures
4. Heavy for caliber bullets
5. Fast twist rate
6. long strings of fire through a hot barrel

When shooting cast bullets in the XCB project, it's very hard to predict because several of these criteria are met, and several are not. We have slow powders, medium to high operating pressures, and long strings of fire, but the XCB cartridges are designed to be inherently balanced, with cartridges that are neither over nor underbore, bullets of the ideal weight for caliber, and very balanced twist rates on the barrels.

Well, it's been a year and a half since I started sending out XCB rifle systems to the participants of this project, and Bjornb has been shooting his "Bertha" rifle and posting groups here with his current barrel since November of last year. He has fired over 2000 HV rounds since that time. Today, the rifle was returned for upgrades and for me to inspect the progress of throat erosion. It has been postulated that shooting bullets made of Linotype and heat treated alloy will erode the throat at an accelerated rate, and this is the first time I have had a barrel come through the shop that has been shot almost exclusively with such alloys and always at elevated velocities.

I brushed the barrel about 15 strokes with a bronze brush sprayed with Hoppe's bore cleaner, then followed it with several patches.
I looked in the throat with my borescope and saw waht looked to be a tar like black substance extending about 1.5" down the bore. It was glossy black, and reminded me of looking in the necks of pulled LC LR brass where they have that black sealant on the inside of the necks.
This black substance extended about 5" up the barrel but only on the crests of the lands.
I scrubbed it many more times with a bronze brush, but this black varnish ignored it.
Finally, I wrapped the brush with 000 steel wool and took several passes down the barrel.
This succeeded in removing the black coating and I was finally able to see what lay beneath.

I saw that the fine reamer scratches had been swept away completely and had been replaced with minute linear striations that are typical.
I saw the faintest bit of gator checking on the ramps of the lands in the throat, and the edges of the lands had been worn smooth, but the angle of the throat was still there and in good form.
The linear striations (obviously caused by shooting, and different from the lapped finish that came from the factory) extended down the barrel about 4".

It's very hard to quantify the erosion of this barrel because it's as different from normal erosion as cast bullets are from jacketed bullets. That said, if this were a 30-06 and I were to guess at how many shots had been fired through it, I would have tentatively said less than a thousand.

Very interesting results. If I were to guess at the supposed barrel life of this rifle using the same loads that got it this far, I would tentatively say it was less than 1/4 of the way to burn out. In other words, this barrel with these HV loads should give 8000-10,000 rounds before wear starts to effect groups.

The black substance is puzzling me more than anything at this point. I spoke with Bjorn about this a while ago and he wondered if it might be the coating on the kernels of the WC860 powder he enjoys shooting in this rifle. I honestly don't know. I've never seen anything like it before. I know that stuff was thick and hard though.

TXGunNut
06-01-2015, 11:34 PM
Did Bjorn mention a decline in accuracy? Wonder if it will improve after you removed the varnish. I'm guessing any upgrades you make will make the second question hard to answer.

Bjornb
06-02-2015, 12:10 AM
There has definitely been a decline in accuracy. I thought it was the shooter (oh horrors), lack of a good front rest, poorly cast bullets, leading, you name it. If you check the shooting posted on both the NOE forum and CB, you'll see that from November 22 (when Tim installed the new Shilen barrel) and until about February, groups were good and consistent. Then started a slow decline. It didn't help that I shot over 300 undersized bullets through the barrel to fireform new brass. Terrible leading ensued, and even though I was able to scrub most of it out, the varnish Tim speaks of was never discovered by me, and was thus never removed.

I try not to make up excuses for my shooting, but I cannot help thinking about the lack of accuracy I've been experiencing lately. Check these groups out:
141105141106141107
Same bullet, same lube, same powder, same primers, same alloy, same mould, same rifle. Even same shooting range. But shot about 2 months apart between December 2014-April, 2015. Sure makes me wonder....

Litl Red 3991
06-02-2015, 06:42 AM
So... looking into the chamber after cleaning, what does it look like?

Generally speaking, most chamber throats that're getting shot out won't look shiny and sharp.

Litl Red 3991
06-02-2015, 06:45 AM
Throat erosion often allows (requires) deeper seating as the round count goes up. Have you checked to see if longer OAL is possible now?

MBTcustom
06-02-2015, 07:01 AM
The throat has moved forward .007 inches.

Mk42gunner
06-02-2015, 07:26 AM
Couple of things come to mind.

1. About the black coating in the throat. I have always read/ been taught that the most common burn rate retardant for smokeless powder is graphite, could that be combining with the bullet lube to create the tar like coating? If so how to prevent it from happening?

2. We have all read that WW760 and H414 are the same powder, could the accuracy results be from lot to lot variation? The picture of the H1414 group looks to have more horizontal dispersion than the WW760 loads.

Robert

Bjornb
06-02-2015, 07:50 AM
Couple of things come to mind.

1. About the black coating in the throat. I have always read/ been taught that the most common burn rate retardant for smokeless powder is graphite, could that be combining with the bullet lube to create the tar like coating? If so how to prevent it from happening?

2. We have all read that WW760 and H414 are the same powder, could the accuracy results be from lot to lot variation? The picture of the H1414 group looks to have more horizontal dispersion than the WW760 loads.

Robert

Yeah we discussed those exact issues. Tim brought up the lube, I thought it could be the burn rate inhibitors. A combo? No clue, but why not? As for the 760/414, I have always been suspicious about the powders being identical, but Hodgdon swears they are the same.

Bjornb
06-02-2015, 07:52 AM
Throat erosion often allows (requires) deeper seating as the round count goes up. Have you checked to see if longer OAL is possible now?

That in itself shouldn't have caused any issues, since I always seated the bullets long and let the bolt seat them into the lands.

texassako
06-02-2015, 08:22 AM
I was thinking the lube was the cause as soon as you said there was a deposit in there. Sort of like the tough stuff you get in a skillet if you get your oil to hot or what happens in your engine's combustion chamber. Something in the lube could be getting a bit toasted with the heat and pressure you are generating creating a carbon build up.

lotech
06-02-2015, 09:05 AM
While the lube may be a contributory factor, it may not be significant in carbon buildup. You might try some JB Paste on an oversized brass brush for scrubbing the affected area only, frequently inspecting progress with your borescope. Or, get a copy of Tony Boyer's benchrest book, "The Book of Rifle Accuracy". He describes in detail his process for doing the job. There are carbon removal products on the market. I've tried several and found them to be ineffective if the carbon buildup amounts to much. Manual scrubbing with a brush and JB is about the only way to remove hardened carbon deposits.

Larry Gibson
06-02-2015, 12:09 PM
lotech's suggestion is worthy of consideration. The deposit is lube, carbon or residue from the retardants on ball powders. It may be a combination of any or all of them. Regardless of what it is it needs to be removed or accuracy will suffer, especially at the higher velocities we are achieving.

I use Hoppe's #9 and about 10 strokes with a bore brush if less than 30 shots have been fired. for 30 - 100 shots I us "Witch's Brew" per the instructions. I use JB's on a patch wrapped around an undersized brush if I've pushed velocities over 2900 fps or fired more than 100 rounds w/o cleaning.

I've come to realize that with the 16" twist barrel best accuracy at HV is coming in the 2800 - 2900 fps range just as I predicted it would before getting the 16" twist barrel. However, it required keeping the barrel clean and free of lead, carbon copper or whatever fouling. With the 14" twist I also use the same regimen.

Notice I mentioned "copper". Don't think you won't get copper fouling from the GCs at these velocities either. I have just patched out the bore after several shooting sessions of 50+ rounds with both rifles and then used Sweet's in the bore. The first patch comes out with blue on it indicating copper in the bore.

I also would bet money that the torture fire forming session had a lot to do with the throat erosion experienced so far. I base that on years of experience watching and measuring throat erosion in match rifles used for rapid fire.

Larry Gibson

leadman
06-02-2015, 01:04 PM
Bruce Hodgdon saw this deposit many years ago when he was selling surplus powders like WC860 and WC870/872. He used BonAmi scouring powder to remove it.
I have used JB Bore Paste to remove it from my Dad's Rem 700 in 7mm Rem Mag left from WC872.

MBTcustom
06-02-2015, 01:11 PM
Bruce Hodgdon saw this deposit many years ago when he was selling surplus powders like WC860 and WC870/872. He used BonAmi scouring powder to remove it.
I have used JB Bore Paste to remove it from my Dad's Rem 700 in 7mm Rem Mag left from WC872.
Great info.
Thank you sir!!!

I shot 75 rounds through the Felix XCB rifle Saturday, 20 of which were WC867. I'll scope the throat and see if I can detect any trace of this residue.

MBTcustom
06-02-2015, 07:57 PM
OK, so tonight I took the Felix rifle and scrubbed the bore with a bronze brush, pulled a bore snake through it twice.
I looked in the throat with the borescope and I saw faint black streaks in the grooves and on the crests of the lands. I'ts the same stuff.
I took the brush that was wrapped with steel wool and tried to scrub it out, but it wouldn't budge. I'm going to try JB bore paste.
If the weather and my scheduled holds, I'm going to take both Bertha and Felix to the range this weekend and see what happens with Leverevolution and House alloy. That aught to be a real sight seeing those two big boys side by side in one rack.
141146
141147

CHeatermk3
06-02-2015, 10:58 PM
If it is hard carbon fouling I suggest trying shooter's choice (Think that's who makes it) black powder gel--it worked for me many years ago in my swift--nothing else seemed to touch it. I think I got the suggestion out of precision shooting magazine

It looked like a grey ceramic coating on the top of the lands.

tomme boy
06-04-2015, 01:52 AM
GM top engine cleaner. If you can find it. It will EAT any carbon. I have not tried it yet on a rifle, but I use a lot of Seafoam cleaner on my outboard motors to get rid of carbon. It works really well.