The flash would be like an anti tank gun...blast would kill everything within a 25yd range.
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Cutting weapons down for lightness and ease of carry was a trademark of the Special Forces in Vietnam, particularly for their "Over the Fence" missions into Laos and Cambodia. One member famously carried a cut down M60 for fire superiority when required, another, a helicopter pilot for clandestine insertions carried a cut down M2 Carbine to hose down anyone charging his chopper and John "Tilt" Meyer of RT Idaho fame turned an M79 grenade launcher into a handgun.
NVA also. I saw a collection of M2 carbines that had barrels shortened, some had sights installed again. Some had the wood cut down too and barrels short like an M1 Enforcer. Some had double loop slings installed to go over your forearms so it would be in place while you handled an equipment bicycle on the Ho Chi Min trail.
Most of these Special Forces cut down weapons were extreme close range break contact weapons only. SF A Teams were generally "light" meaning two Americans and 3-4 Indig or Montagnards. They weren't equipped for extended firefights and when discovered they did a mag dump, threw a frag and 'beat 6' (retreated to the rear) all while calling for an exfil. There was very little ability or need to aim at an enemy when in 6' tall Elephant grass or triple canopy jungle, they simply blasted in the direction of the enemy hoping to slow them down. Further, the SF troopers also carried other weapons such as the XM177/CAR15, K Gun, AK47, and others. John Meyer kept his M79 'handgun' in a custom made holster so it was handy if he needed it but mostly relied on his rifle.