The Keltec Sub-40 (older model, not the 2000) is great. Especally when set up for Glock mags, BUT, it's only the G22 (full size) mags that will fit (and bigger). You can't use a G23 mag unless you have a different (shorter) grip made up by Keltec. I'm not sure if they offer it.
Peep site is funny. You adjust the front site. Still trying to get use to mine. Just have to find the right load I want to adjust the sites to. Also, it fits inside a lap to bag nicely.
The older Sub rifles were aluminum receivers while the Sub 2000 series are plastic. They're trying to make it more wallet friendly. The older Sub Rifles ran $500+ suggested retail. The new ones are below that by a bit.
If recall, the old ones cold be set up for Beretta, SW, Glock and maybe a few more. They use to say that any double stack high cap mag could be made to work. I'm not sure about the new ones. Ya should check out their web page.
(Ya can never have TOO much........)