I have a Ruger Ultralite in .270Win, which has a 20" barrel, but I do not find it objectionable to shoot and I don't think that the muzzle blast is much more than many other rifles. Surprisingly, with Reloader 17, this short barrel comes very close to factory ballistics and shoots quite accurately; more than enough for hunting purposes.

So, I don't think that the short barrel on the M70 Mannlicher would be much different.