I've now built five grenades. So of course now is when I go back to check references, pull screen shots from the Aliens DVD, and see if the current dimensions will actually chamber in a real shotgun (the answer being; "more-or-less.")
I've also committed to building more of them. At least I have some options going forward. If for some reason I end up making more than ten or twenty -- unlikely as that may be -- it turns out the Tormach CNC milling machine at TechShop has a rotating table for the fourth axis. So you could more-or-less use it as a CNC lathe.
Really, though, the basic body shaping is simple enough to do the way I'm doing it. Even a template wouldn't save significant time; I just read the numbers straight from the plan through the DRO and make the cuts. It takes a little time to shape the body due to having to change tools, and a little time just to take down as much metal as has to come off (you can't just hack it off in one pass), but the significant time expenditure really appears to be the internal workings; the part that gives it a spring-loaded button.
For grenade #4, the press-fit plug was slightly undersized. When I started shaping the rear of the grenade the plug began to work free. I tried to bang it out from the button end and mostly succeeded in scaring the body of the grenade with the jaws of the vice. So I drilled it out, which worked a lot better.
I was actually able to chuck the damaged grenade by just the rim, using the four-jaw chuck, and I gingerly took a few thousandths of an inch off. Which cleaned up the worst of the vice scars, and now you can barely tell it apart from the others.
Grenade #5 I managed to create start to finish in under three hours. I was working a wee bit too quickly, though, and ended up cutting the button almost 2/10 of an inch too long. This is another difficult spot in the grenade; the exposed length of the button is determined by the depth of a blind hole drilled in the unfinished body, with any errors in that long drilled hole compensated for by adjusting the distance to the corresponding shoulder on the button. In the case of grenade #5, I added the error instead of subtracting it. And I was in too much of a hurry to measure the button before I seated the plug.
Which is a long way of saying the average construction time still has to be estimated at around five hours each.
I did one for fun. I did a second one because there were some errors in the first. Any more, I do for strictly mercenary reasons. So I have no compunction about wanting to get paid for my time. Depending on how I chose to amortize the tooling I bought just for this project, and by what number to divide the (large) minimum order of plastic caps, the fixed expenses work out to $14 to $23 per grenade.
Pricing out the labor cost of mailing, communicating with buyers, dealing with returns, etc. is beyond me. If I was pressed, I'd estimate another $5 to the assumed costs. And $6 postage within CONUS.
That leaves the rest as more-or-less hourly. Or as a "service," if you want to calculate musician-style. This number doesn't break $10/hour until I hit sixty bucks a finished grenade. And that's just the metal-work; the plastic cap adds a not-insignificant time investment on top of that.
So as much as I'd like to come down to $45 per grenade, I don't think I can do it.