FWIW The 18 have a problem shooting in the colder temps here in Central NY. I know that some people have even tried 20 inch twist barrels with good results down south
The bullets I made Ed were on the .925 jacket which is very scarce these days. The current jacket I use are 1.020 match and work out pretty well with 17 and 16 inch twist rates.
Thanks on the insight to Hart 18 twist barrels I'll have to ask Jack Sutton about that . I gave one away because , when the temperatures dropped to fall weather here,
the rifle would not shoot even with the .925 jackets. I believe that the 18 is marginal as stability, it locks you in to a specific weight,
In my earlier post I wrote about different twist rates that we used and different bullet weights. 5 shot groups in the low 100.s are nothing to sneer at.
While I agree, that 0.1xx groups are nothing to sneer at, if a 1:18" twist barrel - or, even 1:19" twist - does not shoot .925 based bullets, in any condition (Temp.), it is not due to stability (twist) - it's on the DYNAMIC side of the equation - something other that the spin rate.
Again, at sea-level, and standard conditions, for .30 Cal. X .950" long FB bullets, the 1:19" twist rate produces Sg 1.5. For comparison, the long-standing 6MM model - .825" jacket/68 Gr. (finished length between about .845-.850" for FB bullets), and 3300 FPS , through a 1:14" twist barrel, results in Sg 1.3 - now, there's a weak model . . . about where the 18.5 twist would land us with a .30 Cal. bullet about 1.050" long.
While Mr. Sutton would be a good reference, it's just as easy to fit a tight patch to a jag and mearure YOUR barrel - especially a button-rifled barrel, which tend to vary somewhat. All of the Hart barrel users, to whom I have sold bullets, have advised me something like, "it's close to 18.5". These are excellent barrels, but, if/when the twist rate exceeds 1:18" - even a little, are best suited to bullets based upon the .925" long jacket. When working with a Hart barrel, I always recommend bullets based .925" jackets - historically, for me, weights between 100 and 123 Gr. - thus paired, Hart 18's have always
shot.
For thirty cal. bullets of up to 1.050" long (Sg 1.4), the 1:18" twist is the SLOWEST desirable twist rate: normally, FB bullets based upon the 1.00" long jacket, and depending upon ogive configuration, finish at lengths between .990" and 1.015" - well short of the 1.050" which results in Sg 1.4 - thus are assured of more than adequate stability. Essentially, assuming a twist rate NO SLOWER than 1:18", for a LARGE safety-net, it's best to limit bullet length to 1.02" or shorter. I've shot my 18 twist PacNor barrels, using 1.020" long FB bullets, at temps well below ZERO F, without experiencing disappointment . . . this at 1,000 FT above sea-level and about 30% RH.
As Charles points out, for a given jacket length, within the range of useful mass ( the range which will afford precision bullets), stability is LARGELY about the LENGTH, NOT the weight. Oh, and I almost forgot - again - who could have predicted the great jacket shortage, thus which weights/lengths/twist would be good/ill choices???
Keep 'em
ON the
X! RG
