Okay, I use the Lyman 20ga. and 12ga. overgrown airgun pellet wad-slug designs and have got them to produce good to excellent results for my needs in these respective gauges. I have, however, not been able to produce an effective slug load for the .410 that will fill my needs when used in a smoothbore gun. The best load I have been able to develop to date for the .410 consists of substituting a .338 caliber 300gr. (which is 11/16oz.) round-nose, flat-base bullet for the shot inside the wad. When the bullet is loaded backwards and the tail and nose are reversed the resulting wad-slug is slightly nose-heavy and thus the slug flies straight and true out to about the 25-yard line and produces some respectable groups. Beyond that range, however, the proverbial sh*t hits the fan and slugs start tumbling and keyholing. This combination is simply not balistically stable enough without rifling spin to stay flying straight for long. Yah, every ten rounds or so you'll have a volunteer which hits the target straight and very close to the point of aim but that don't mean nothing in a hunting load. Now, I have got this particular load to work like a charm out of the rifled .410 slug gun I built for my little brother using a .303-brit action and a .405-Win. barrel blank. Prints nice tight groups all the way out to 100 yards and he has taken several deer with it in the shotgun/muzzle-loader/handgun only local hunting section. Due to the lower power of this gauge he keeps shots at 70yards or less and does quite well.
So, I have come to the conclusion that the only way I am going to be able to produce a heavy and slow slug load for the .410 that is stable out of a smooth bore is to have a custom mold cut. Yah, I know it's going to cost me $$$.$$ but as of right now the option is on the table. So I have been figuring and doodling and figuring some more and this is what I have come up with for a tentative prototype for a 5/8oz. SWC .410 wad-slug design:
The first picture (all of which are central cross sections) shows my tentative prototype wad-slug design beneath a conventional factory 1/4oz. slug. The second picture shows the respective complete slug loads.
Now, I'm figuring that the first question out of chute is going to be something along the lines of, "Why are you making a 5/8oz slug for the .410? Isn't that like way too heavy of a slug for that gauge?" Well, actually no. Let's go back to shot loads. For the .410 in the 3" shell length there are 5/8oz. "high-velocity" loads with a muzzle velocity of 1,200fps., standard 11/16oz. loads with a muzzle velocity of 1,100fps., and heavy 3/4oz. loads with a muzzle velocity of 1,000fps. Now when you substitute an equal weight wad-slug for the shot in a shot load you end up with a slug load that has slightly higher velocity and slightly lower pressure then the original shot load. Why? Because under acceleration in the bore loose shot has fluid like properties that result in increased pressure and thus friction on the walls of the guns barrel. The wad-slug, however, because it is a single piece of solid lead does not exert this same additional pressure and friction and thus you get the slightly higher velocity and lower friction. Nice trade off, huh !!!
So, it is entirely possible for the .410 platform to launch a 5/8oz. (275 grain) wad-slug at a muzzle velocity of 1,200fps. But why would you want a heavier bullet moving at a slower speed and the rainbow like flight path that goes with that combination? Answer, because this combination is much better at retaining nock-down power over extended range, and for a shotgun slug load respectable hunting ranges from 50 to 100 yards are for all practical purposes extended range. The biggest, baddest, hottest factory .410 slug load on the market right now pushes a 1/4oz. (109gr.) RN slug out the end of the barrel at about 1,800fps. with a muzzle energy of 784 foot-pounds. That is enough energy for humane and respectable hunting use, however, guess how much of that energy it's still packing at 100 yards? A measly 332 foot pounds, which is woefully inadequate for all but the best placed (or lucky) shot. Now, how much energy would my 5/8oz. wad-slug design still be packing at 100 yards --- a very respectable 664 foot pounds. In fact a standard factory 20ga. slug load consisting of a 5/8oz. RN lead slug leaving the muzzle at 1,600fps. has an impact energy of 752 foot-pounds at 100 yards. That means that in terms of energy a heavy .410 slug load is nipping at the heals of a 20ga. load of the same weight. In fact at even greater range (about 200 yards) it will actually surpass the 20ga. slug load in terms of energy retention performance!!! Not that it would be ethical to take a shot at such range with a smooth bore gun, but this is indeed what the numbers show.
Now for those of you who think I pulled those numbers out of my, errr where the sun don't shine, here is a link to a respectable, accurate, and easy to use ballistic calculator on the net:
Here are some figures you will need to know (verify if you wish). The diameter of a .410 slug is of course .410 (although most factory slugs are slightly undersize by a few thousandths). The diameter of a 20ga. slug is 0.615 same thing applies about factory ones sometimes being slightly undersize. As far as weight is concerned 1/4oz. = 109.375 grains (109 to keep things simple), 5/8oz. = 273.4375 grains (275 for simplicity). Muzzle velocity of factory .410 slug = 1800fps, for my prototype = 1200fps, and finally for standard factory 20ga. = 1600fps. Factory slugs are round-nosed and my design has a SWC nose design.
Now, back to my tentative prototype slug design. First off it has a very deep hollow base cavity as well as a healed bullet style base. Both serve to make the slug very nose heavy and thus maintain aerodynamic stability during flight regardless of any spin imparted by the rifling or lack thereof. Secondly, since the vast majority of .410 shot wads available to reloaders such as ourselves are designed for use in both 2-1/2" and 3" shot shells they only enclose the bottom 2/3's or so of the shot column (or in this case wad-slug) when used in a 3" load and the forward 1/3 or so is exposed directly to the bore. My tentative prototype design takes this into consideration and the bottom portion of the slug is sized at about .350" to tightly fit inside the vast majority of .410 wads. The forward head section of the slug is sized at .410 to ride the bore of the gun and is equipped with lube grooves. These lube grooves consist of a series of deep and wide grooves separated by thin bands. This is done so that if the slug is fired through a gun equipped with a choke these thin bands are easily swaged down by the choke (easily compared to a solid head design). Thus the fact that the slug is full bore diameter does not present a hazard when fired in a choked gun. The nose design of the slug is basically that of a SWC with a very shallow angle. This is a direct off-shoot of the nose design used on the Lee "Modern Minie" and "R-E-A-L Bullet" that are designed for use in muzzle loaders at very similar velocities to that of shotgun slugs and have a nose angle optimized to give maximum accuracy and good performance on game at these velocities. For further information on this particular nose style reference page 119 and 200 of the Lee 2nd edition load manual.
So, I guess that's about it for the blab from me. I need some input from more experienced casters out there on my design. Comments and input on the general design and logic behind it? How thin can I make those bands on the head and how thick do I need to make the cylindrically shaped base to get the mold to full out okay with my intended alloy (soft range lead plus a little tin --- about 40/1 alloy)?