G.R.T. [Gordon's Reloading Tool]

[jamesharrison]

For those who have played around with G.R.T., have you used its OBT calculator?

If so, have your groups tightened?

I've used it for a number of years now, but have decided to delve into it further. I'm playing with it for both 6.5 CM & .308 W.C.F., and using the OBT function, it's telling me to adjust my powder charge by 0.03 grains [43.97 grs. vs. 44.0 grs.].

Considering, Luddite that I am, that I use one of my 3 Ohaus [RCBS] 5-10 scales, I'm not really able to cut my powder charge down by 3/100th of a grain, but am interested to know if you use it & followed their recommendations, did it make a huge difference in your groups?

Back in the day ['80's & early '90's when I shot Benchrest], I would be appalled at the group shown below [quite a few are >0.500" except for the nowadays inevitable pulled [jerked] shot in a 5 shot group, as I have developed Essential Tremors so this is likely as good as it's going to get from here on out]. Based on my shooting notes as to wind conditions that day, I would attribute the bit of vertical & the horizontal to the switching wind conditions last Thursday [L > R (N to S, switching to gusts from the West - shooting due East].

My wind flags are currently gathering dust in the garage as I haven't dug them out in almost 30 years; then again, I haven't used wind flags when hunting EVER!

 

[Hitzy]

I can't tell what distance you are shooting at there...that would help.
My bad...
100 yards, wind is not going to play into it with any 6.5 or 308, maybe 22lr...

 

[L.Parratt]

OBT has been pretty much debunked by modern Ballisticians like Litz and Williams.

Today really the best way to reload is through large sample size testing, quality brass, and Bullets.

Buy some Berger Hyrbid Ogive bullets, seat to your mag length. Throw it in some Lapua/Alpha/Peterson brass and you're done.

You can pretty much predict a guns potential accuracy through a Calculation, developed by Bryan Litz.

Muzzle Energy (ftlb) ÷ Weapon Mass (lb) ÷ 200 = expected MOA

 

[Hitzy]

OBT has been pretty much debunked by modern Ballisticians like Litz and Williams.

Today really the best way to reload is through large sample size testing, quality brass, and Bullets.

Buy some Berger Hyrbid Ogive bullets, seat to your mag length. Throw it in some Lapua/Alpha/Peterson brass and you're done.

You can pretty much predict a guns potential accuracy through a Calculation, developed by Bryan Litz.

Muzzle Energy (ftlb) ÷ Weapon Mass (lb) ÷ 200 = expected MOA

And shoot at 300m minimum, 100 is not going to tell you anything.

 

[mimile]

In GRT, OBT works well when all your input data is correct, including case capacity, barrel length, twist rate, temp, jump and even more importantly your measured velocity (not the one calculated by GRT). 0.03 grains is insignificant. Your groups at 100 yards look great, more than enough if your purpose is hunting.

 

[jamesharrison]

OBT has been pretty much debunked by modern Ballisticians like Litz and Williams.

Today really the best way to reload is through large sample size testing, quality brass, and Bullets.

Buy some Berger Hyrbid Ogive bullets, seat to your mag length. Throw it in some Lapua/Alpha/Peterson brass and you're done.

You can pretty much predict a guns potential accuracy through a Calculation, developed by Bryan Litz.

Muzzle Energy (ftlb) ÷ Weapon Mass (lb) ÷ 200 = expected MOA

The rifle is being set up for hunting, so would not use Berger bullets, including their hunting line, although I use them exclusively in my .223. I'm all out of Remington 165 grain PSPCL bullets and have had good luck in the past with Hornady bullets on game, thus my choice here.

The problem with large sample size testing is primers, actually lack thereof. [To remedy that, I've ordered 200 Starline SRP brass]
I'll look into that other brass; thanks.

Re: Litz calculation. That calculation does not hold up, given that LV [10.5#] & HV [13.5#] rifles, shooting 6 PPC, even 30+ years ago when I shot Benchrest, were usually capable of substantially beating that predicted number, unless wind or mirage got completely out of control. My log books for both my Shilen LV & my Hart HV rifles were full of match-winning targets of less than 0.571" [the predicted accuracy using the above formula], including a fair number of screamers [5 shot groups measuring <0.100"].

Re: shoot at 300m minimum, 100 is not going to tell you anything.
Considering you cannot see past ~75 yds. here in the Shield where I hunt, I'm not sure what benefit that would be.

I realize my loads are more than adequate for hunting purposes; I'm approaching this as more of a theoretical problem/challenge, as back in the day there were, at least that I was aware of, no analytical tools for predicting interior / exterior or terminal ballistics.

After measuring my actual case capacity, actual barrel length, actual bullet length, 1 calibre seating depth, choosing the powders I have on hand, QL came up with the loads, which was then cross-referenced with GRT. The target photos shown are after ~200 rounds since I cleaned the barrel. I clean sporter rifles once per year, after hunting season.

Only after I have chosen a load for accuracy, do I chronograph the load at different temperatures. I can then do a drop chart & commit that to memory, even though it has been a very long time [it's been almost 40 years since I lived in the NWT where every shot was a long one] that I've had a long shot at game. Typically, my shots at Canada geese are longer than my shots at white-tail or bear.

Please bear in mind this is a ~40 year old Whitworth Classic [based on a Yugo-produced/England-finished commercial Mauser 98] sporting rifle, with a 22" light sporter, almost Featherwight, barrel [muzzle diameter 0.550"]. It has been glass-bedded & a Timney trigger installed, but otherwise it is in stock configuration. It wears a Swarovski Z3 3-9x36 scope sitting in Leupold QRW rings on top of Warne steel bases.

Thanks everyone for their suggestions. After I have a chance to grab some more components, I'll update my results.

 

[junkdude]

OBT has been pretty much debunked by modern Ballisticians like Litz and Williams.

Not familure with the debunking work, however I am cautious of anything where the word debunk is used.

I have used ladder load development method quite a bit and it has always produced exceptional results. I believe the basis for it working so well is linked to the OBT idea. I've followed up using pressuretrace testing and it seemed to confirm (or rather support) the resultes from the ladder load data.

Coincidence? Perhaps.

Besides, can't think of a good reason to used an large sample size (to be statistically correct) when smaller sample will produce good results.