Throat erosion, firelapping and cast boolits

[crabo]

I asked a noted cast boolit mold maker about firelapping a rifle to get better cast boolit performance. I am worried about premature throat erosion. Here is what he said about keeping the erosion from happening.

"You don't, but it is minimum if you use cast bullets only, and have the hardness close to 12 bhn. The little throat wear is good for both jacketed and cast, in fact almost mandatory for cast. Maybe you aren't aware of it, but hot cast loads wear the throat as fast as hot jacketed loads. The erosion is from powder, not the bullet, and is least with ball powders. Hot stick powders are worst, with the slowest burning somewhat better but not equal to ball powder."

What do you think about his opinion? I have no experience with cast in rifles other than 357 leverguns.

Thanks,

[357maximum]

I do know for a fact that large doses of ball powder in a 357max @ 45,000PSI will "MOVE" the throat ever so slowly forward...


.I thought stick powders did this less than ball powders.....now I am confused and will have do go do some reading.

[docone31]

I used IMR4227 in my .357 Supermag. The top strap used to get flame cut every time I fired it. After a while, it had a noticeable cut. I eventually sold it, but I could see it grow after every match.
I could sure get some flat primers with that load!
I had a couple of small pistol primers mixed in with small magnum primers. Those were flat, and the primer case actually cratered to the firing pin hole on one.
Never had a blow through though.
That was one tough pistol.
Wish Dan Wesson still made it.
Never noticed forcing cone erosion though.

[felix]

It is the result of the flame temperature. Hotter the temperature, the more nitrogen gets forced into the metal (microscopically). Once the steel has been hardened, it begins to flake/clump like vertisol clay (gumbo) when dry. The friction of the passing projectile then can rip off the steel because it has cracks which can be grabbed by an expanding and passing projectile. This is the erosion. Nitroglycerin actually makes worse heat (hotter and longer) than straight up nitrided fibers. Inorganic chemicals added to powder tend to lower the peak heat, and thus help erosion, but, in contrast, hurt fouling. No matter what, loads over 30K start to become obvious offenders over time. HiVel and Cordite have been taken off the market because both would generate unwarrented heat. Both had 40 plus percent of nitroglycerin. ... felix

[Boomer Mikey]

The information you received is correct; generally, ball powder burns cooler as it has more coatings to control burn rate and those same coatings make ball powder difficult to ignite and keep lit.

Most cast bullet loads don't generate enough pressure to worry about it as accuracy will usually go away at those 45-65K psi pressure levels.

The SuperMags, 454's and bottleneck case smaller calibers with large case capacity in particular suffer the most.

New gun barrel throats, grooves and lands typically have machine marks that a few hundred jacketed rounds, some careful hand lapping or fire lapping should take care of before optimum accuracy potential can be reached.


Boomer :Fire:

[Larry Gibson]

That is the short and long of it. As Boomer said though most cast loads don't generate the pressure because accuracy goes away at those pressure/velocity levels. Barrel life is thus long and most wear if only cast bullets are shot is not noticed. Those who really shoot a lot of bottleneck rifle cartridge loads with medium or slow burning powders may see some barrel wear.

Larry Gibson

[357maximum]

MOST and USUALLY...glad them two words were included...

[Boomer Mikey]

MOST and USUALLY...glad them two words were included...

I knew I'd get flamed If I excluded your barn burning loads Michael...

Some of the guys can push the envelope by paying careful attention to bullet fit, hardness, design, and powder selection to reach jacketed bullet velocities and pressures with accuracy.

Have fun,

Boomer :Fire:

[crabo]

I guess one of the big things that I am not clear about, is if I firelapp my rifle, will it errode the throat to where it might not shot jacketed well?

Thanks,

[Boomer Mikey]

Read Veral Smith or Marshall Stanton's book... Why do you think you need to fire lap in the first place?

"Fire lapping" is a procedure or method used to accelerate barrel wear using abrasives... a lack of experience - proper application of this process can result in a worn-out, useless barrel. When used properly, the procedure can produce a good shooting cast bullet gun that otherwise could only shoot jacketed bullets with a reasonable degree of accuracy.

Constrictions exist in barrels where they're roll stamped with company logo and/or caliber/safety warnings. Constrictions also exist where dovetails are cut for sights and stock mounting studs, and where threads are cut or rolled and tightened into receivers/frames. Rough grooves exist in barrels where the rifling button "chattered its way through the barrel due to a dull cutting edge, a chipped tooth, and/or lack of proper cutting fluid and cooling. Rough tops on rifling lands exist because a deep drill had a chipped edge or it was fed faster than it could cut the hole without proper cooling and cutting fluid supply or the finish reamer had similar issues.

Pushing a lubricated; over-sized, pure lead fishing sinker/lead ball/bullet through your clean barrel from one end to the other will provide you with a good idea of what kind of condition the bore is in. When done properly, you'll "feel" every tight spot (constriction/fouling), rough spot (scale/machine tool chatter marks etc.) This, along with pure lead oversize slugs taken from the muzzle and breach and a casting or pound slug of the barrel chamber and throat will provide you with an excellent idea of what kind of condition your barrel is in and what (if any) work is needed to improve the bore's condition.

If the bore slug passes through the barrel smoothly from one end to the other with even resistance you're lucky indeed; and, if the muzzle and breach slugs measure the same groove diameter within 0.001" you're golden... even if the groove diameter is a couple thousandths larger than normal at both ends you should have a good shooting gun. Most likely though, the chamber end will be smaller than the muzzle end (thread constriction) and you'll feel tight spots inside the bore when the slug passes the front sight dovetail slot or soldered sight base, roll stamped surfaces and where the barrel is screwed to the receiver/frame. Even worse... is when that slug feels like it's being filed down by the bore when you're pushing the slug through and the tops of your barrel's lands look like a corncob.

Fire lapping can smooth a rough bore and/or throat to lessen copper or lead fouling and to prevent gas cutting caused by bullets passing through a barrel constriction. An oversize BHN 10-12 bullet impregnated with abrasive is pushed through the bore with a light powder charge at 500-600 fps. This will polish tool-marks and erosion/hard powder fouling from the throat and/or remove a small amount of a constriction in the barrel. If the medium soft bullet passes through a constriction it's swaged down in size and won't spring back to its original size to continue polishing beyond the constriction. Each additional abrasive impregnated BHN 10-12 medium soft oversize lead bullet through the bore will remove a little more of the constriction or rough surface. Cleaning and slugging the barrel frequently during this process provides you with a method to gage your progress. The oversize BHN 10-12 lead bullet will reach the bottom of the grooves and the top of the lands to work on all the surfaces in the barrel's bore... the kits using jacketed bullets do nothing for rough barrel grooves and won't remove constrictions as hard copper jackets spring back and continue cutting.

Hand lapping is another process used to improve a bore's condition... usually to put the finishing touches on new premium quality barrels or clean up the throat of a good barrel to extend it's service life.

A good cleaning and scrubbing with JB bore cleaner is another way to prep/accelerate breaking in a new barrel/cleaning/removing hard fouling/scale from the throat and bore of a barrel with minor issues.

Boomer :Fire:

[BABore]

The easiest way to tell if you need to FL a cat boolit shooter is to slug it. Twice. I use hollow egg fishing sinkers per Marshall Stanton's FL book. The hole through the center gives the lead some extra area to flow into rather than spring back slightly which leads to false readings. Pound one slug from muzzle to breech with a heavy hammer using inertia to move it, not small taps. Note any constrictions or hesitations in the movement and mark there location(s). Then, most importantly, is to pound another sinker flush or just below the muzzle, then pound it back out from the breech. This slug should be the smaller of the two. If it's not, you have bore constrictions. If your ok with this, fine! Go get new cylinders for all your wheeler where the chamber throats are smaller than the barrel's groove diameter cause it'll have the same effect with cast. If the full pass bore slug is only 0.0002 to 0.0005 smaller than the muzzle slug, I usually will pass on fire lapping unless other needs dictate it. Hard cleaning, or slight leading when it shouldn't normally. Yes firlapping will move a throat some. On the 8-10 guns that I've done, the most movement was 0.027". It may seem like alot, but in alot of cases it's from removal of rough machining areas that are best removed anyway. I have always found that firelapping has improved my accuracy. In some cases, very profoundly, cutting group sizes in half. I do not consider the Tubb's FL kit, with hard jacketed bullets to be the way to go for bbl constrictions. It may be fine for a final polish or touching up a throat, but that would be it IMO. Jacketed bullets wear on the bore and do little for the groove condition. Hard cast boolits have too much spring back to gently taper a bore. Straight AC'd WW's or equivilent 9-12 bhn hardness boolits are the way to go. You also need to do it a airgun velocities of 400-600 fps. Done right, and not overdone, it will improve your cast boolit accuracy. I wouldn't recommend that anyone tackle it without thoroughly reading either Veral's or Beartooth's book on it.

[crabo]

I have firelapped all of my pistols, revolvers, 22 rifles and my 357 lever gun. They all showed improvement after lapping. I have never done a bottleneck cartridge rifle and I just bought a 308 bolt gun. I plan on shooting cast in it in the near future.

I was apprehensive about "shooting" the throat out by firelapping. The statement I quoted was from Veral when I asked him about firelapping the rifle.

BTW Boomer, that was a nicely done explanation of the firelapping process. That post would make a good sticky for people who are not familar with firelapping.

[Boomer Mikey]

[QUOTE=crabo;445865] I have never done a bottleneck cartridge rifle and I just bought a 308 bolt gun. I plan on shooting cast in it in the near future.

I was apprehensive about "shooting" the throat out by firelapping. The statement I quoted was from Veral when I asked him about firelapping the rifle.
QUOTE]

I Like my Savage 308 scout rifle a lot. The bore was great and I ran maybe 10 firelap rounds thru it to smooth the machine marks a little and it's been great. Try the RCBS 165 Silhouette and RCBS 200 SIL bullet if you like heavy bullets. The RCBS 150 GC is good too, but the RCBS 165 SIL is the best... place a thin piece of flue tape on the bottom of the mold blocks to "Beagle" the nose large enough to engrave the lands and you can shoot the 165's beyond 2300 fps. Your rifle's 1:12 twist rate should help you get there as well.

http://castboolits.gunloads.com/attachment.php?attachmentid=8294&stc=1&thumb=1&d=1218006189http://castboolits.gunloads.com/attachment.php?attachmentid=8273&stc=1&thumb=1&d=1217875029

Get yourself a Redding 308 taper crimp die... it'll lower SD's.

Have fun,

Boomer :Fire:

[andrew375]

It is the result of the flame temperature. Hotter the temperature, the more nitrogen gets forced into the metal (microscopically). Once the steel has been hardened, it begins to flake/clump like vertisol clay (gumbo) when dry. The friction of the passing projectile then can rip off the steel because it has cracks which can be grabbed by an expanding and passing projectile. This is the erosion. Nitroglycerin actually makes worse heat (hotter and longer) than straight up nitrided fibers. Inorganic chemicals added to powder tend to lower the peak heat, and thus help erosion, but, in contrast, hurt fouling. No matter what, loads over 30K start to become obvious offenders over time. HiVel and Cordite have been taken off the market because both would generate unwarrented heat. Both had 40 plus percent of nitroglycerin. ... felix

You are right about the effects of nitriding on barrel steel. But not about the influence of high levels of Nitroglycerine or cordite. First of all there were as many distinct types of cordite as there are of conventional powders, with varying amounts of NG, all higher than modern propellants. The flame temperature was actually cooler with cordite than with equivalent Nitrocellulose powders. The BIG difference though, as far as barrel erosion is concerned, is that whilst the combustion gases from NC powders are oxygen poor (and nitrogen rich) NG rich propellants like cordite produce oxygen rich gases. That is that with NC all the oxygen is used up before the fuel, whereas with those containing NG all the fuel is burnt before it runs out of oxygen. So with NC you have lots of free nitrogen atoms floating around to bond with the iron on the barrel surface to form Iron Nitride (as used to coat cutting tools) and with NG lots of free oxygen to combine with the barrel steel to make iron oxide.

Both Iron Oxide and Iron Nitride are what are known as intermetalic compounds and are very hard and very brittle, exactly what you don't want inside a barrel. The problem was further exacerbated with cordite due to it being combined with corrosive primers.

Cordite as a small arms propellent ceased to be used due to a combination of manufacturing, handling and loading problems. Not the least of which was actually getting the stuff into the cases. With bottle necked cases, like .303 british, the cordite was loaded with the case still as a straight cylinder, then the case was formed around the cordite. This is why pre WW2 british ammunition all feature gently sloping sides and shoulders; no more annealing was possible.

[Bret4207]

My advice woudld be to put a few hundred rounds through the gun before lapping of any kind. I did a lot of the lapping at my Dads shop. It's dull work and can make a barrel far worse than it started out to be if done wrong. It can also do worlds of good for other barrels.

Shoot it a bunch before you lap, throat or make other changes. Clean it really well too if it's had a lot of jacketed through it, especially if you want to do cast. Each gun is an individual and there's no one answer to the lapping question, at least in my opinion.

[felix]

Way to go, Andy! Thanks for the addendum. ... felix

[Boomer Mikey]

My advice would be to put a few hundred rounds through the gun before lapping of any kind. I did a lot of the lapping at my Dads shop. It's dull work and can make a barrel far worse than it started out to be if done wrong. It can also do worlds of good for other barrels.

Shoot it a bunch before you lap, throat or make other changes. Clean it really well too if it's had a lot of jacketed through it, especially if you want to do cast. Each gun is an individual and there's no one answer to the lapping question, at least in my opinion.

I agree with you Bret,

It's much better to shoot a good barrel better without any help from the peanut gallery and reserve fire-lapping for barrels that are otherwise unserviceable for cast bullet, or even jacketed bullet use.

My experience is that a very rough bore with serious copper fouling issues can become serviceable with greatly reduced fouling characteristics after a surprisingly small amount of lapping. Once fouling is under control, normal break-in procedures will to provide a barrel with long service life that's easy to clean.

I don't advocate fire lapping to improve accuracy in a good barrel; however, as a means to make an otherwise (to me) useless barrel serviceable, fire lapping has improved accuracy considerably.

Boomer :Fire: