Confusion about groove diameter: .3543" 9mm barrel

[Njal Thorgeirsson]

Hello,

I just got a replacement barrel for my Glock 26 to shoot cast boolits (yes, I know you can theoretically use the factory one, but I just like the idea of the traditional rifling and non-bulged cases for easy reloading).

Of course, the first thing I did was slug the barrel. I slugged the barrel 3 times and measured each (of 3 possible) groove diameters (barrel has 6 lands). All 3 slugs came out to, as far as I could tell, .3543" with a high-quality calibrated micrometer. With calipers, the muzzle certainly measures less than .355". All three slugs felt tight in the barrel.

Is .3543" unusually small for a 9mm barrel? I realize that I can size my boolits to .356" and probably be ok. But can I shoot .355 jacketed bullets through it safely?

Thanks!

Edit: I also forgot to ask: Will I likely experience trouble sizing .362" boolits to .356"? I haven't sized a boolit before, as I am waiting to purchase a sizing die based on what my barrel slugs at. So I'm not sure exactly how much pressure it will take to size an air-cooled 98% WW : 2% Tin alloy by 6 thousandths.

[DLCTEX]

That seems really undersize to me. Sizing a boolit .006 may cause the lands to collapse some. I would try some to see. A method to support the lands is to size and lube the boolits to .360, then to .356. The lube will support the lands hydraulically. This is sizing base first. Nose first sizing will require a tight fitting push rod to avoid flashing on the bases.

[fryboy]

9's tend to vary - alot , your's sounds great for a new barrel and j-words , perhaps breaking it in with them will smooth it out ( and up ) ?
the alloy you list is what i'd term medium , not real hard but not too soft , if you water quench/heat treat i'd suggest sizing them as soon as possible to make it easier , less tin and antimony will make them smaller of course ( i cant get the thought out of my head that for a mold designed for a 9 mm that's one fat mold .... and would be fatter with say hardball or #2 alloy )

[Njal Thorgeirsson]

9's tend to vary - alot , your's sounds great for a new barrel and j-words , perhaps breaking it in with them will smooth it out ( and up ) ?
the alloy you list is what i'd term medium , not real hard but not too soft , if you water quench/heat treat i'd suggest sizing them as soon as possible to make it easier , less tin and antimony will make them smaller of course ( i cant get the thought out of my head that for a mold designed for a 9 mm that's one fat mold .... and would be fatter with say hardball or #2 alloy )

Well that definitely makes me feel better. I guess I should mention that the boolits are a bit out-of-round and are .3605-.3620. It is the Lee 125 grain round nose. I guess I'll try sizing them as is (pan lubed to retain the groove if possible), straight from cast to .356". From what I've read, i assume a boolit sized .0017 over groove will likely be ok for leading?

The barrel is supposedly SAAMI-spec (it is KKM Precision), and SAAMI lists groove diameter as a minimum of .355". I suppose 7 ten-thousandths less isn't anything to be concerned with. By the way, the visible quality of the KKM barrel is exceptional.

[fryboy]

should be but remember that while sizing is king for preventing leading it's just one part of the equation ( the most important part but ...) i dont know how lyman did it but many of their loads were listed at actual bore size ( ermm wait i think the #2 alloy helped them in that regard or the straight lino they used in other applications ) the barrel it's self actually dictates what it prefers but usually .001 - .003 over works well , most of my nines have a .355-.356 bore and i use the same sizing die that i do for my 38's and 357's ( .358 ) thankfully that diameter also fits in the cases and my various chambers and simplified my sizing a bit !! i have this mold but havent tried it in a 9 yet ( it's one of my newest molds ) mine's been casting around .3587-.359 -ish so i dont have the radical sizing problem you're presented with but the pan lubing should save the grooves so that aspect is covered , sizing them that much can of course soften them a wee bit but as long as you dont exceed your alloy strength you should be good to go

[Njal Thorgeirsson]

Great! Thanks for the info!

I guess I'll cross the sizing logistics bridge when I come to it. I was mainly concerned about the bore being too small for jacketed and/or .356 cast.

I guess if people have luck with .001-.003 over groove, .0017 over groove should be a fine start.

FYI, I'll probably pan lube with Barry Darr's before sizing, then tumble with 45:45:10 after sizing just for good measure. I guess this info should get me to the experimentation stage. Thanks again for the help!