Someone modified a 1911 45 round drum to work, too. I'm not searching for the pics. Check in the archives.
"It isn't always being fast or even accurate that counts, it's being willing. I found out early that most men regardless of cause or need aren't willing. They blink an eye or draw a breath before they pull the trigger and I won't." - The Shootist (John Wayne)
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!" - Patrick Henry
I saw a modified drum for the 45 carbine on youtube a while back, but the shooter had issues with it feeding properly. Do you know if they ever got it to work? From what I understand, the drums don't work very well even in the pistols they were designed for, so not sure if they will do any better after modification for the Hi Points. I would be interested if you know of somebody who actually did a mod that worked with them though.
At this time Hi-Point does not make a high-capacity magazine for any of its carbines or pistols. To my knowlege ProMag has the the only 15 rd magazine available, but it is only for the 995 (9mm) and its reputation is spotty at best.
The only option I have heard of is the 15 rd 1911 magazine. Like a standard 1911 mag they could be modified to work in the 4595 but I have no idea of its reliability.
I did a conversion on one of the 15-round ProMags over the weekend. I used the info from the youtube video. The mag that Paul uses appears to be a Triple K mag. My next conversion I will use it instead of the ProMag. My conversion worked, although I haven't gotten to fire from it yet. It function tests just fine, however, the steel in the ProMag is really stiff. I had considerable more difficulty making the conversion than Paul in the video did. So, maybe I just had problems or the Triple K mag is softer/thinner steel. I dunno. But it is fairly simple to do if you watch the video. Here is a link to the video: