Umarex Hammer bullet question-Accurate Molds designs?

[Greg_E]

So I am pricing out some options, and looking for feedback. I'm wanting around 600fpe and as much velocity as I can get. Looking at the Umarex page, they suggest that this can be had by some style of Lyman design in the 330-350 grain range. Of course they don't specify which design, so I'm left questing for choices. The best choices are from Accurate, the most number of designs, weights, and size control all custom cut into a mold of choice.

What I'm looking at is a 2 place mold, with 2 designs, unless someone tells me different. #1 is a 355gr round nose with lube grooves in .510 (+.002 -.000) and a boat tail 375gr in the same size/tolerance.
https://www.accuratemolds.com/bullet_detail.php?bullet=51-355R

https://www.accuratemolds.com/bullet_detail.php?bullet=51-375Q

Wise choices? Dumb choices? Get something lighter or heavier for one of the sizes?

I already have a Lee 450gr with lube grooves that I think I can make work, and a 320gr muzzle loader that I could make work by altering the mold. Don't want to alter the mold because I may buy a muzzle loader kit, and having the ability to use that ammo would be nice. But I don't think it will work for the Hammer due to the tapered drive rings being too small diameter.

Or am I reading the attached chart wrong and it is not a Lyman mould, but a Lyman alloy being used? What I'm looking for is what they show for the 330gr lead(Lyman) entry. Or something better.

[Spacebus]

I would look for an accurate mold with the most possible bore contact in the 300-350 grain weight range. From my testing with different ammo on my AEA .45 Bullpup the more bore surface area in contact with the slug the higher the velocity. Accuracy at 50 yards seems no different comparing a one driving band slug to almost .5" of surface length contacting the bore, but velocity/efficiency is much higher with the increased contact area. My hypothesis is that the long bore seal has a similar effect to a longer barrel since the slug is still sealing to the bore while exiting the barrel. In cartridge based guns a longer bearing surface creates higher chamber pressures, I'm not 100% sure that directly translates to air guns, but it seems to.

[Greg_E]

In that case, the second option (boat tail) should probably work fairly well, the smooth side and almost half an inch of length in contact should yield a good seal.

The only reason to shy away from it is the research that Bob has done with his boat tail designs, these almost always try to reduce barrel friction. Definitely will need to powder coat this boat tail to get any level of lube on the outside, I don't think tumble lube will do much of anything.

There is also a spire point round in about 400 grains over on NOE that was in stock a few days ago. Not sure if the BC would be higher on that round than on the boat tail.

[Spacebus]

In that case, the second option (boat tail) should probably work fairly well, the smooth side and almost half an inch of length in contact should yield a good seal.

The only reason to shy away from it is the research that Bob has done with his boat tail designs, these almost always try to reduce barrel friction. Definitely will need to powder coat this boat tail to get any level of lube on the outside, I don't think tumble lube will do much of anything.

There is also a spire point round in about 400 grains over on NOE that was in stock a few days ago. Not sure if the BC would be higher on that round than on the boat tail.

I think that less surface area can be better in lower power air guns, especially in smaller calibers. Maybe a longer slug with separate driving bands is better than on long bearing surface.

For ballistic purposes I pretend my AEA .45 is actually a super heavy magnum revolver using subsonic ammo, as the muzzle energy and velocities are similar. Ammunition for .457 magnum subsonic loads favors a long bearing surface with a fairly large meplat, I think this is for  a reason. In .457 rifle rounds they tend to be very long with a huge bearing surface, maybe even a boat tail, but these are usually fired at supersonic velocities. My NSA slugs are this shape, yet yield similar velocities to a shorter, lighter slug despite the lighter slug also having only one driving band and a boat tail. With the velocities we are dealing with on air rifles, I think a larger meplat is going to be better for hunting, but maybe a long range the pointy NSA style slugs may be better. My next mold will be from Accurate and have at least three different shaped cavities.

[Gut2Fish]

I don't know what the barrel twist is in the Hammer. The advantage of boat tails in air guns is the makers usually set them up with too fast of twist for the power they have. Resulting in a need for longer and heavier slug for accuracy that are slow. The boat tail not only provides more physical length at less weight it also produces more length effect by design of laminar air flow. Bottom line is boat tails require faster twist than a slug of same length. If a problematic barrel like the Airforce .457 that requires very heavy slugs then using a boat tail aids in accuracy with higher velocity. I've not heard much about the accuracy or twist of the Hammer. But it's basically a 50 yard hammer that doesn't need high accuracy so maybe the point is mute.

At any rate, there is a decent designed slug I see at Accurate that's 350 grain. Has a large meplait and gas check. Gas checks work near a boat tail and you can also have the mold altered for a .460 base and voila, a boat tail. 51-350VG Another popular slug is epp/ug for short range, large meplait, killing. Accurate 51-250B is an EPP/UG design.

[Greg_E]

Airguns of Arizona lists it as 29.5 inch 1:24 twist barrel. https://www.airgunsofarizona.com/big-bore-airguns/umarex-hammer-.50-caliber/

I think a 250 grain is going to be too light for the power I'm looking to get. High 800's to middle 900's FPS is the speed I'm looking for, no desire to be sonic and then fall almost immediately to sub-sonic.

I think I'm going to order tonight, probably the two designs shown above. But I need to look up some definitions first to make sure I'm ordering the correct diameters.

They show the diameter of the lube grooves as .515 but also ask about the body diameter, is this the same or different? If different then I may need to wait until the rifle comes back from service so I can measure the lands to let the "nose" ride in the bore. I didn't think to measure before I sent it back for warranty, and I'm now regretting this. Groove diameter is supposed to be .510 so lands are probably near .500 or maybe down to .495

[Spacebus]

Airguns of Arizona lists it as 29.5 inch 1:24 twist barrel. https://www.airgunsofarizona.com/big-bore-airguns/umarex-hammer-.50-caliber/

I think a 250 grain is going to be too light for the power I'm looking to get. High 800's to middle 900's FPS is the speed I'm looking for, no desire to be sonic and then fall almost immediately to sub-sonic.

I think I'm going to order tonight, probably the two designs shown above. But I need to look up some definitions first to make sure I'm ordering the correct diameters.

They show the diameter of the lube grooves as .515 but also ask about the body diameter, is this the same or different? If different then I may need to wait until the rifle comes back from service so I can measure the lands to let the "nose" ride in the bore. I didn't think to measure before I sent it back for warranty, and I'm now regretting this. Groove diameter is supposed to be .510 so lands are probably near .500 or maybe down to .495

I doubt your lands are .495". In my .457 Challenger the lands are .457 and the grooves are .461" I would imagine your lands are .510" and your grooves are no more than .514"

[Greg_E]

I measured the diameter in the magazine and got right at .510 so the grooves will be near that value. Guess I better wait until it returns from service so I can measure. I have more ammo already cast than I can possibly shoot in one day with refills happening every 3 shots and taking 15 minutes for the compressor to refill.

[Greg_E]

I went ahead and ordered both of the above in .510, hope at least one of those works out.