5.7 vs 5.56

Discuss rifles chambered for the 5.7x28mm; P90, PS90, and AR57.
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mel023
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5.7 vs 5.56

Post by mel023 » 27 Nov 2011, 04:12

I'm new to this site so please forgive me if this subject has been covered elsewhere. If it has, I would appreciate it if some kind forum member would point me to that other thread.

I have a 5.7 upper for my AR lower receivers but I'm not too terribly happy with it. I think it's way too heavy for the caliber and those long 50 round PS90 magazines are basically pointless as the range I shoot at has a 10 round limit on loaded mags on the firing line. I've looked at the new(?) Excel Arms models (at least on-line) but none of my local gun shops seem to be very interested in carrying them. One shop even told me that they won't carry them because they can't get any kind of dealer price from Excel so they don't have any room to make a deal with their customers. It's all full retail pricing from the Excel website.

However, that same shop carries the Kel-Tec rifles so that got me thinking. Are the 5.7 and 5.56 cases dimensionally close enough such that converting something like the Kel-Tec SU-16CA would be workable? I know the 5.7 is shorter than the 5.56 but what about mid-case diameter, rim diameter, shoulder angle, neck length and all the rest of it. Of course, I'm presuming it would require a new barrel because the chambers are different.

I would appreciate input on the viability of this idea and how to go about doing it.

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Rapier1772
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Re: 5.7 vs 5.56

Post by Rapier1772 » 27 Nov 2011, 11:20

I don't normally link people to wikipedia but they do have the case dimensions listed on the right side, sans shoulder angle. Short version, the case diameters/lengths are different
http://en.wikipedia.org/wiki/FN_5.7%C3%9728mm" onclick="window.open(this.href);return false;
http://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO" onclick="window.open(this.href);return false;
Image


If I remember right, you can order from Excel & they'll ship to your FFL. I think that is the only way to get one of their guns right now.
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mel023
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Posts: 9
Joined: 09 Nov 2011, 01:52

Re: 5.7 vs 5.56

Post by mel023 » 27 Nov 2011, 14:10

Thanks Rapier. Seeing the two cartridges side by side like that really puts in perspective. It's obvious that they are way too different for my idea to work. Too bad, it seemed like such a good idea at 3:00AM. Maybe I should try going to bed earlier. All I have to do now is sell my AR5.7 upper in order to by an Excel X-5.7R

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