Diameter per coat?

[waksupi]

I picked up another Trapdoor this week. Accuracy is exceptional for three shots, then I would get fliers as the barrel heated a bit. It appeared I may have a slight bit of tipping of the bullet on target.

I wonder how much diameter is gained by doing multiple coats, and would it be strong enough to hold up to the firing? I suspect I will be experimenting with this when I get time.

[Dragonheart]

I asked this same question a number of years ago to a Phd professor in polymer physics in our discussing the 50K psi chamber pressure of a 30/06. Since the average depth of rifling is .004", he suggested a .003" build out of the polymer. He said the polymer would indeed hold its bond and withstand the torque of spinup. The polymer also seals and insulates the core alloy from the momentary heat at ignition.

I personally have fired small caliber PC cast bullets in excess of 3500 fps so I can tell you I have not found a velocity limit due to the polymer. I have also fired large caliber PC cast bullets in 50K range of chamber pressure and have not found a pressure limit due to the polymer. So the Phd was right as far as I am concerned because I do not own anything faster or larger to test.

I double coat rifle bullets using a partial cure first coat. Good luck on your testing.

[waksupi]

I asked this same question a number of years ago to a Phd professor in polymer physics in our discussing the 50K psi chamber pressure of a 30/06. Since the average depth of rifling is .004", he suggested a .003" build out of the polymer. He said the polymer would indeed hold its bond and withstand the torque of spinup. The polymer also seals and insulates the core alloy from the momentary heat at ignition.

I personally have fired small caliber PC cast bullets in excess of 3500 fps so I can tell you I have not found a velocity limit due to the polymer. I have also fired large caliber PC cast bullets in 50K range of chamber pressure and have not found a pressure limit due to the polymer. So the Phd was right as far as I am concerned because I do not own anything faster or larger to test.

I double coat rifle bullets using a partial cure first coat. Good luck on your testing.

Thanks for a very good answer!

[MrWolf]

That was an impressive response Dragonheart. Thanks

[Drew P]

Hitek adds about .0003 per coat

[tmanbuckhunter]

I asked this same question a number of years ago to a Phd professor in polymer physics in our discussing the 50K psi chamber pressure of a 30/06. Since the average depth of rifling is .004", he suggested a .003" build out of the polymer. He said the polymer would indeed hold its bond and withstand the torque of spinup. The polymer also seals and insulates the core alloy from the momentary heat at ignition.

I personally have fired small caliber PC cast bullets in excess of 3500 fps so I can tell you I have not found a velocity limit due to the polymer. I have also fired large caliber PC cast bullets in 50K range of chamber pressure and have not found a pressure limit due to the polymer. So the Phd was right as far as I am concerned because I do not own anything faster or larger to test.

I double coat rifle bullets using a partial cure first coat. Good luck on your testing.

Then this leads me to one question. I see it mentioned all the time... don't size after coating, size after coating. I noticed my clear PC on a .460 diameter bullet usually adds about .003 to the diameter. I was sizing that back down to .460, but should I not bother? Bullets with a gas check I obviously have no choice.

[Dragonheart]

Some don't size at all; I see absolutely no advantage in doing this other than a time saver. Bullet diameter is going to vary so your pressures as well as accuracy . Some size the bare alloy, then size again after coating; I don't see any need for adding an extra step, but I have done no testing to confirm that.

A properly cured polymer is going to be well bonded and harder than the alloy. Sizing is moving the metal and making the bands round. Unfortunately the remainder of the cast bullet is still lopsided just as it came out of the mold, the PC does not fix this. Nose sizing in a separate step does nothing to correct this either.

I size after coating and pay particular attention to the base of my bullets. Nose first sizing is much faster for me, but if the moved metal is being pushed downward and deforming the bullet's base consistency is going to be a problem. Sizing base first tends to provide a flatter and more uniform base. This seems to change somewhat with a particular mold and bullet design, but for rifle I size base first and handgun I usually go for speed and size nose first.

Since most molds are not nose pour the sprue cut hole creates a non-uniform bullet base, cooking PC bullets base down the polymer tens to fill in the cut hole and makes for a flatter base. There is no reason I can see to gas check a handgun bullet, but I can see a perfectly set gas check as an accuracy advantage for a rifle, problem here is setting a check perfectly flat and perpendicular.

This is just my take on the process.

[Conditor22]

If the boolit doesn't chamber/go into battery after coating size it; if it does your golden.

You will find that there some guns where the throat will not accept a boolit .002 over the bore's diameter and need to be throated to shoot cast accurately.

[white eagle]

if you are after perfectly flat base's why would you no use a gas check
they aren't a sure fix for all base's but there is an advantage in some

[Dragonheart]

if you are after perfectly flat base's why would you no use a gas check
they aren't a sure fix for all base's but there is an advantage in some

For handguns I have many loads developed using plain base PC bullets that will group under 1-1/2" at 25 yards. Very few hand gunners can shoot better than that, so to go to the expense, time & effort of a gas checking handgun bullets makes no sense to me and I think most would agree. And that is assuming the gas check actually added any improvement to begin with.

As far as PC rifle bullets the polymer creates a jacket and does everything a jacket does. Once again I have full power loads that give excellent short range accuracy without a gas check. But as the distance increases the lopsided cast bullets start to wobble and once it starts it only gets worse and so does the accuracy. A gas check does not fix a lopsided bullet and neither does PC. Choosing a better design for a bullet, like a bore rider, and sizing it to the barrel makes a big difference and this is where having a perfectly flat base would be an asset. "Bama" has achieved full power long range accuracy doing this, but his bullets are far from just dropped out of a mold and PC.