GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: PakProtector on June 24, 2015, 09:41:17 AM
-
What are the land/groove measurements on the 26" twist, 30 Pellet barrels Bob and Sean designed? I recall the spec being .300 groove, and .294 land...
I have two in hand, one significantly older and a recent one and they are both significantly larger than the spec, though both very close to each other.
cheers,
Douglas
-
So, given no responses from others wid this barrel, I'll ask this an other way. Is .0016" over the nominal groove diameter spec reason enough to return for an other try?
cheers,
Douglas
-
Douglas,
I have several of these barrels and I don't remember them being oversized. I'm headed out on holiday but will be back mid next week, I can slug one then and post the results.
Tom
-
Thanks, that would be great.
cheers,
Douglas
-
Douglas,
I just received a couple of the 30 cal airgun barrels from TJs, but have not prepped them in any way so I can't slug them and get a measurement that I would trust. Yes, they are speced at .300/.294, 1/26. It has been a few years since I've bought any barrels from TJs ( and I have bought a bunch of them in the past) and I see that the muzzle end is now clearly marked with a sticker. I guess that has to do with the direction that the mandrel is pulled through?
Give me a few days and I will prep the barrels and slug them and see what I get. Honestly though, I feel that there is always some amount of spring-back from any lead slug, even from soft pellets and I never trust my measurements 100%. The pellet would almost have to fall through the barrel if it had no spring-back. Although I know that slugging the barrel is the accepted method, what about making a custom gage pin and polishing it down to achieve a "gage fit" to check the land, and then a second pin similar to a fixturing diamond pin that is relieved such that it has 2 contact-fins to check the groove dia? The groove gage could possibly be a blade, I guess. A lot of trouble, but I would be more willing to trust the measurements made with gages. But that said, your 0.0016" is certainly not insignificant.
Lloyd
-
I've been wondering about just this.
As I'm building to an already existing Bob/Sean TJ's .300 liner,
and it's my first .300 -
with no .300 BBT's available anywhere for sale cast, even to slug it;
I need to know precisely before spec'ing and investing in a mold and swage.
I have nothing vaguely resembling internal measuring equipment to those tolerances!
Anybody got (or know where can get) a few cast pure lead .300 BBT's to sell me so I can slug this barrel?
Thanks,
Rod
-
I finally did a little checking on one of the .30 cal barrel liners that I have. A .295 dia pin would fit in the bore, a .2955 would not. I chucked the blank into a 4 jaw chuck on my lathe and used an indicator with half thousandth divisions, and a .078 ball tip, to get zero runout inside the lands. Because of the wide grooves and narrow lands, it was not too difficult to get a very repeatable "bump" over each land. Each bump was consistently 2-1/2 thou. Using the equipment mentioned, the measurement on the one sample barrel blank appears to be about .295 land and .300 groove, or maybe a half thou larger. That is about as accurately as I can measure with the equipment on hand.
-
Hey what about going to Midway USA and buying some Cerrosafe and following the directions that come with it?
-
rotometals.com sells a similar(or rebranding for cero safe?)that that costs 15.00 a lb.
cheap enough to not reuse(save a cast of every barrel!)
-
Chuck and John, Thanks for the additional info. That Cerrosafe and the 158-190 rotometal alloy do look like the same thing. The shrinkage/growth vs time chart for the Cerrosafe is interesting. With larger calibers the timing could make a difference.