what determines driving band size?

[lawboy]

Okay, I have two, 300-grain 45 moulds, one is a Lyman 454629BM made for the Freedom Arms 454 Casull. The other is an RCBS 45-300FN. Both bullets are Gas Check designs. I like the bullets dropped from both of them as I shoot a SBH 45 and a 94Trapper Carbine in 45. However, I cannot help but notice that the designs of these two flatnose bullets are very different in the driving band area. The RCBS mould has two lube grooves and a shallow crimp groove. It also has four relatively thick driving bands. The Lyman mould has three lube grooves and five rather thin driving bands. Why the difference in design for two bullets that are essentially intended for the same purpose? I mean, the Lyman mould is intended for the higher velocities of the 454 whereas the RCBS mould was likely designed around the 45 Colt but, both are gas check designs so obviously high velocities were contemplated when designing both bullets. I just don't understand the pros and cons of the two approaches.

[Bass Ackward]

I just don't understand the pros and cons of the two approaches.

Beautiful thing is that you don't have to understand. Just like that old hair commercial that only your hair dressor knows for sure, only your gun needs to know the difference.

But honestly now. Think of the variables.

1. Rifling height, the numer of them, and what percentage of the bore they occupy. Drive area is drive area. Shallow lands require more land area to run the same velocity.

2. How rough is the bore and how long is the barrel? Narrow bands allow more lube. You might even add what quality lube are you going to use? The better the lube quality, the less you need. The longer the barrel the more lube you need.

3. What type of powder are you running? BP requires more lube to keep fouling soft than smokeless does.

4. What powder speeds do you want to run. Wide bands take more time to displace metal raising pressure with the same bullet hardness. Narrow the exact opposite. So are you a fast powder guy wanting to shoot cheap or a slow powder guy wanting all she's worth?

5. What bullet hardness do you want to run. Wide bands are strong so you can go softer on your mix.

And the list goes on. So again, you, the reloader, establish all the conditions you want and the gun is forced to decide. You just observe the results.

[9.3X62AL]

Good info, Bass. This text almost rates a sticky--let's see where it goes.

[lawboy]

Thought provoking.
After posting this thread, I went shooting.
I took along my Bisley 45 Colt with 4-5/8 barrel that I just got back from the gunsmith who shortened it. I took along two loads:
19.0grs 2400 under the Lyman 454629BM bullet with Winchester primer; and
19.0grs 2400 under the RCBS 45-300FN bullet with Winchester primers. Bases were mixed. Lube was RCBS Green stick lube. bullets were sized to .452. I did not use gas checks on either bullet.

Now, the gun performed beautifully with the RCBS bullet, ringing the 100-yard 12-inch and 9-inch gongs like there was no tomorrow as long as I broke the shot right.

Not so with the Lyman bullet. I was getting bullet strikes in the dirt 50 yards short of the target, then some15 feet above the target on the berm, others 20 feet to the left or right. It was down right comedy, only not very funny.

So I thought about it, and thought about it. I also shot the load from my Winchester trapper. It shot a little better from the carbine -- I could see almost all of the shots hit the berm behind the gong, but I did not hit the gong once.

I thought about it some more. I concluded that maybe the thin driving bands were stripping out due to the fairly heavy load and relatively soft alloy I use. I am not really convinced this is the problem, however.
What is going on?

[DOUBLEJK]

Looks like ya found yer answer....use the RCBS...:-D
Course that Lyman would be fer days ya wanta seek n maybe ya'll find...:-D

[montana_charlie]

I can't say what causes the differences you observed.

But, one more piece of general information about wide versus narrow driving bands on bullets of the same basic shape and length...

'Narrow bands' often means more bands (and therefore) 'more grooves'. 'More grooves' increases aerodynamic drag. More drag reduces ballistic coefficient. Lower BC usually means poorer long range performance.

But sometimes, it doesn't happen that way...
CM

[StarMetal]

I believe I've told this before. Gun: Smith & Wesson Model 25 Caliber: 45 Colt Barrel lenght: 8 3/8 inches. Bullet: RCBS 255 gr SWC Alloy: Various to aircooled WW's, to water quenched WW's, to straight linotype, and everything else inbetwee. How does it shoot: One of best revolvers I've ever owned.

Now here's what it does with cast. I dig up alot of my bullets when I can. Cast bullets in this revolver strip worse then I've ever seen. There is only (usually and depending on what power loading is used) as little as 1/32 inch of bullet that isn't stripped and keeping it from making a total 360 degree strip. But yet, this revolver is deadly accurate. Ask 44 man. Puzzles me that it can shoot as well as it does and strip that bad. Absolutely nothing in the barrel but powder carbon. This is a revolver that I've never ever had anykind of leading issue with. The only think I notice about the rifling is that it's very very very shallow, exactly like rifling with be for a 45acp and Smith maked those too, so I surmise they just use one rifling machine set up to do both 45 Colt and 45acp. I've never got stripping with any 45acp's i've ever own/owned, even with very hot loads. The bore and groove on that Smith are tight, it's not oversized. It also seems to gobble up anything I feed it. In a way it reminds me of my 30-40 in load friendlyness. But it strips cast bullets. So I pay it no nevermind and enjoy shooting it. I have three friends that have this model also, two are 6 in barrels and the other a 4 in. They all shoot as good so saying I have the longer barrel is a good thing or the bullet surely would never get a grip on the rifling isn't an answer.

Joe