Why does a Gill-gun (6A/87A) have gills? Here is why.

[RickN]

Thanks. I appreciate that. It's been in the family for as long as I can remember and I'm 63. My older brother is 71 and it's been around as long as he can remember, too. The malfunction has always been present so when we shot it we pushed the bolt handle in and used it as a repeater. I'll give you a shout one of these days.

If anyone can fix it, he can.

 

[El Pablo]

Here is the answer. This is a selected section of the actual patent explaining the function of the gills.

When no specific means are provided for the escape of this blast of gas, the fouling, that is to say, the mixture of bullet grease and powder residue is forcibly blown back into the various parts of the action adjoining, and to the rear. The accumulation of this fouling in the parts of'the action will, in a relatively short time, cause the various parts to stick and fail to function properly,
In a gun of this type, that is to.say, a semiautomatic or automatic blow-back gun, this condition renders the gun inoperative, or at least unreliable. It is, therefore, advisable to provide as generous a means of escape for the gas and the fouling carried thereby as possible.

I have accomplished this by making the opening on the left-hand side consist of a series of substantially vertical slots, having such a width in relation to the thickness of the wall of the receiver that no gas or particles of fouling can be directed to the rear at an angle of less than about 45 degrees with the axis of the gun, which gun axis is the basis of all the angles herein mentioned. This permits the necessary escape of the fouling without the possibility of any of it .being directed back into the shooters face.


Here is a link to the full patent documents on Google patents.

US2290156A - Firearm - Google Patents

View attachment 432836


This includes Savage models 6 and 7, Stevens/ Springfield models 85 and 87, and various other store brands.

Rookie, it’s so they can breath under water.