Tuesday, March 13, 2012

More HI Point drum shenanigans

Well, I got the formed feed lips welded on, and I was pretty happy with the way they looked and it sure seemed like they would perform well. Of course, it wouldn't be the Hi Point drum project if it didn't give me a headache somewhere......

I am still having troubles with the rounds nose diving and slamming into the feed ramp. I epoxied some plastic inserts into the drum that helped cut down on friction tremendously, and helped the rounds feed through the drum and into the tower more reliably, but it still isn't allowing for enough spring force to keep the rounds pointed up enough to work reliably. I finally in a fit of frustration broke out the CAD program and got to work on a couple ideas. It didn't take long for me to find a couple areas where I was making some simple mistakes that may be costing me time and money.
In designing this whole drum and mag tower project, I kept thinking that the 9mm cartridge is a solid diameter throughout it's case length, and had I stopped long enough to really investigate, I would've seen that it is not. It actually varies about .01" in diameter over the length of the case. This may not matter in a magazine of 10 rounds with a stiff mag spring, ala the factory Hi Point magazine, but in the case of this drum project, where spring tension is less than optimal even with a fully wound center spring, plastic wear inserts, and a decent mag tower, every little bit of force matters when it comes to getting the bullets to nose up as they travel out of the long mag tower. Since the cases are just slightly tapered, by the time the rounds get up the top of the mag tower, there are new issues.
1. the cases are pointing downward because the .01" taper/per round adds up
2. The original spring force that was sufficient to dump the entire magazine before is not a linear force by the time it reaches the top of the mag tower, i.e. it is not equally distributed across the top face of each round. Instead of a straight upward force, the force is now working at odd angles due to the inherent angle of the mag tower as well as the taper of each additional round. To add to that, the mag tower angles forward, and the taper of each consecutive round is also "forward leaning", creating a real alignment issue once the round gets to the feed lips. For illustrations' sake, here is a drawing I worked out real quick in my CAD program. There are no force vector diagrams or anything like that, but it should be pretty evident where the problem lies.

As you can see in the drawing, the angle of the rounds leaving the drum is fairly flat with respect to the drum, but as the rounds travel up the magazine tower, they begin to tip downwards. The difference over 12 9mm cases is severe enough that it will keep this entire project from working correctly until I fix this error.
Now, with that said, the top of the mag tower is on a 180 degree plane, and the top round is pointing just slightly (~3 degrees) upwards, as it should be (technically I think the rounds point even further up from the factory mags, but 3 degrees is enough to work). The mag tower as pictured here is the same as a factory mag, with a 17 degree angle with respect to the 180 degree plane. The angle of the drum in this picture is roughly the same angle as will be needed in real life for proper performance, and it is pretty evident that the angle of the rounds traveling up the mag tower will probably result in some friction. The rear of the case will be in constant contact with the inside-rear of the mag tower. I may be able to glue in a strip of delrin or PTFE strip .005" thick and maybe that will solve any friction issues.
When I first built the drum mag about 9 months ago, I got it to work somewhat well, and that was when I had a mag tower with about a 20 degree angle in relation to the drum. My most recent attempt had the drum on a 90 degree plane in relation to the gun, and didn't work anywhere near as well. It seems that this drawing is calling for at least a 36 degree angle, and possibly more than that if I really want it to feed well.
I hope to have time over the next few days to bend up a new mag tower; I've gotten pretty adept at building those lately =/

So, the new feed lips looked great, were strong as heck, and had the mag tower been to spec, I might have had more success. Since I was too lazy to map this whole thing out until now, I get to build another mag tower and try again. Keep checking back for updates as I get to them......


  1. Sounds like this may be a breakthrough. Good luck!


  2. Yeah, if I could just get the desire back to tinker with it. I'm pretty aggravated about it right now.
    However, I'm sure I'll get back to it shortly, next week more than likely.