The John Unertl Company mainly made telescopes for target shooters. The length is a function of the optical system. Another feature is that the adjustments are external and the tube is free to slide in the mounts. When the rifle is fired it moves to the rear leaving the scope forward. The scope must then be pulled back into battery for the next shot.
At the beginning of WW2 the USMC looked at what was available on the US rifle scope market (not much) and decided the Unertl was the best choice. An order was placed for about 3500 units which was subsequently cancelled after about 1750 were delivered. Of that number perhaps 1000 to 1200 were assembled onto various M1903 match rifles. The reason the order order was cancelled was theat the USMC Commandant had received reports that the "rifle was not effective in combat". A quantity of M1903A4's was thereafter requested from the Army. The Corps never threw anything away however so with careful maintenance the scopes soldiered on thru Korea and Vietnam on 03's or Winchester Model 70's.
Its length, finicky adjustments, delicate construction, lack of moisture resistance, sliding mounts, narrow field of view and low relative brightness made it far from an ideal combat scope especially in the jungles of the South Pacific.
The Unertl did have fairly high magnification at 7.8X which was certainly an advantage over the scopes selected for the M1903A4 and M1snipers.
The Unertl Company went on to design and manufacture a new 10X scope for the Marines in the 70's for use on the M40 series. This was a compact, heavy duty scope with internal adjustments and rigid mount.
Regards,
Jim