What is the correct amount of freebore for accuracy in a pistol barrel?

[Ruger 15151]

I have a 9mm Storm Lake barrel that requires me to load JHPs very short in order to keep them off the landes when chambered. I have sent them to a gunsmith to have it throated so I can load longer cartridges.

I included a dummy load so that he could cut the landes just enough to give me about .015 clearance off the lands to allow for variations in bullet shape and case length. However, he says he has a custom ground throat finishing reamer that creates about .180 of 1 degree freebore at the end of the chamber. He says that this allows the bullet to completely leave the case, and center it before it enters the riffling.

Since this is a target barrel, will this large bullet jump have a negative effect on accuracy?

 

[243winxb]

I would study SAAMI standards for the 9mm before doing anything. http://www.saami.org/specifications_and_information/index.cfm

 

[entropy]

Freebore isn't measured in degrees, it's in thousandths of inches or millimeters. Perhaps you mean the angle it sets on the beginning of the lands? I doubt that would be .18 degrees, .18 degrees is 3.33333 minutes of arc. I suspect the lands wouldn't reach the full height they were cut to by the time they reached the muzzle.
If the freebore is .18 inches, that shouldn't degrade the accuracy near as bad as the above modification would.

 

[Ruger 15151]

Freebore isn't measured in degrees, it's in thousandths of inches or millimeters. Perhaps you mean the angle it sets on the beginning of the lands? I doubt that would be .18 degrees, .18 degrees is 3.33333 minutes of arc. I suspect the lands wouldn't reach the full height they were cut to by the time they reached the muzzle.
If the freebore is .18 inches, that shouldn't degrade the accuracy near as bad as the above modification would.

Thank you for your response.. but not quite sure what it means.

Yes...Freebore is measures in inches. It begins right after the chamber and ends at the riffling. It varies in length depending on the barrel manufacturer. It is generally .001 bigger than the bullet and tapers into a leade. Special reamers are ground specifically for the amount or freebore you want as well as the taper the leade you are looking for. In my case, the pistol smith is looking to ream .180 of freebore after the end of the chamber and have it start .001 larger than the bullet diameter I have chosen and taper the leade 1 degree.

I know Wilson Combat loads one of their 9mm Signature Match Ammunition (125gr HAP JHP) at 1.150 COL. My current Storm Lake barrel will only allow me to load that same bullet at 1.030 without touching the lands. Therefore, the match shooters that are using this Wilson Combat ammunition are having their barrels throated with a longer freebore and tapered leades than factory standard in order to be able to chamber these rounds .

I'm just looking to see if anyone else can share their experience.

FYI...Here is some helpful reading posted by a well knows retired Match pistol builder in the north east.... "The 9mm barrels that set and held the National Record for 10 years, had a .100 long, bullet diameter +.001 freebore into a 1-1/2 degree leade. Those were Kart 16 twist barrels.. I am now using faster twists but with the same reamer design. The freebore and long gentle leade will be very forgiving as to COL..within reason."

 

[entropy]

Mea Culpa. I misunderstood the data because it was a bit ambiguous. The barrels you descibe at the bottom sound like they'd be accurate indeed.

 

[Jim Watson]

Freebore is generally straight cylindrical with the leade into the rifling at a slight taper. But it is his reamer so I guess he can call it what he wants to.
One degree is very shallow, there are some 1.5 degree rifle barrels out there with no freebore.
I wouldn't worry about SAAMI, when it gets into chamber throats there is a lot of variation. As long as your ammo chambers freely, it will shoot safely. Will it be more accurate? I dunno.

 

[Ruger 15151]

Jim.. Thanks for the reply.

I agree. 1 degree is pretty shallow.

Again, my initial purpose was to ramp the leades enough to load my 125gr JHPs at 1.120 - 1.130. Since I currently can only load that bullet at 1.030 in my Storm Lake barrel without touching the lands, can that be done by just ramping the lands? not sure.

Do you have any thoughts as to a better way to accomplish this?

BTW - I live on the anti-gun west coast so good pistol smiths are hard to find and the ones that are here are burred in work for the next 2 months. Do you have any recommendations?

 

[Jim Watson]

You could ask Storm Lake.
I know another brand of aftermarket barrel will throat for the customer's ammo, maybe Storm Lake will, too.

 

[243winxb]

I am thinking the location of the ogive (where the curve reaches full bullet diameter) is more important then the COL, as long as it fits the magazine.

 

[Jim Watson]

The key measurement is where the ogive reaches bore diameter, or in the OP's gunsmith's case a diameter matching the diameter of the tapered "freebore" at the desired OAL.

 

[Ruger 15151]

Thanks everyone! I really appreciate the information you have taken the time to provide.

 

[jmorris]

For me distance or "jump" to the lands is not the main factor in 9mm pistol accuracy for me. I can take ammunition that I can only shoot in a pistol with a 2" group at 25 yards and shoot it in a rifle with much better accuracy.

At that point I can conclude that the ammunition is better than I am. I guess at least I loaded it...