Head space and chamber issues with my NoI MkIII

[Kanaskichuck]

I wouldn't mess with it, . I own a no1 m111 Parker Hale supreme drilled and tapped with parer hale scope mount and rings with an old 3x9 Bush, the gun still has the original rear sights and fitted with a sprter front sight. Ishapore 1917. With factory winchester super x 180 soft points I can shoot 3 shot 1" groups @ 100m. I also hand load , neck sizing is all I do with my 303's lee sells the kits. Fired 303 casings only need to be cleaned up, trimmed, neck sized, primed, use loading data. I use sierra load data.

 

[tiriaq]

I wouldn't mess with it, . I own a no1 m111 Parker Hale supreme drilled and tapped with parer hale scope mount and rings with an old 3x9 Bush, Ishapore 1917. With factory winchester super x 180 soft points I can shoot 3 shot 1" groups @ 100m. I also hand load , neck sizing is all I do with my 303's lee sells the kits. Fired 303 casings only need to be cleaned up, trimmed, neck sized, primed, use loading data. I use sierra load data.

When factory ammunition is displaying that odd brass flow at the mouth, there is something amiss. A .303 rifle should not display this sort of thing.
Trimming cases might be s solution for handloading. If well trimmed case do not exhibit the brass flow at the mouth, then case length & chamber dimensions could be where the problem is.
There appears to be a problem in the chamber. It is not a factory barrel; heaven only knows who reamed the chamber, with what reamer..
There are different .303 gauges. Service have .064 GO, .074 NO GO, which is more a Field gauge. SAAMI adds a .068 NO GO.
OP hasn't indicated the gauge set used.
In addition to the gauging of the rifle, rim thicknesses of .303 cartridges vary. This affects the relationship between the cartridge and the rifle, what is all lumped together as "headspace".

 

[lledwod]

The rifles been mailed to a gunsmith. I'll update when I know more. The plan is to turn it back and ream the chamber. Thanks for all the knowledgeable and thoughtful responses!