need help .308 load

[strobes]

Hi, I have tested two loads last weekend and today.

1. .308win 147Gr FMJBT,Win p/n WB762147N) 43Gr Varget - 2812FPS - they shoot tight groups @100
2. .308win 180Gr Super-X SP,Win p/n WB308SP180) 39Gr Varget - 2622FPS - the same tight groups too @100.

And load #2 always is 3.5' below load #1. I can reproduce this consistently.
My question is how it's possible to have 3-4' difference between groups shot using two types of bullets @100

 

[scout1]

The answer to tour question is RECOIL and BARREL TIME
But I normally see my results in the reverse of your observation.
The lighter bullets traveling faster, exit the muzzle before the rifle barrel reaches the top of its recoil swing and hit your POI if you zeroed your scope to that particular load combo.
The heavyer bullets moving slower, exit the muzzel higher in the recoil swing normally causung a higher impact.

BTW, I must have an extreamly slow batch of Varget. I'm using a Nosler 168 BTHP over 45 grns and only getting 2690 out of a 26' barrel

Hope this helped

 

[Mikem]

Im with scout on this one. I shoot my 24' barreled 308 with 168g SMK using 44.5g of Varget for 2701 fps, and its always in the sub .5s for 5 rounds at 100 yds.
You may want to recheck that load, something is wrong.
Mike.

 

[NateHaler]

Hi, I have tested two loads last weekend and today.

1. .308win 147Gr FMJBT,Win p/n WB762147N) 43Gr Varget - 2812FPS - they shoot tight groups @100
2. .308win 180Gr Super-X SP,Win p/n WB308SP180) 39Gr Varget - 2622FPS - the same tight groups too @100.

And load #2 always is 3.5' below load #1. I can reproduce this consistently.
My question is how it's possible to have 3-4' difference between groups shot using two types of bullets @100

Uh, the FMJ Boat Tail, going nearly 200 fps FASTER, will of course shoot higher than a 180 Soft Point,crummy aerodynamics, comparatively).

That's how.

 

[strobes]

Uh, the FMJ Boat Tail, going nearly 200 fps FASTER, will of course shoot higher than a 180 Soft Point,crummy aerodynamics, comparatively).

That's how.

Yes I was thinking of a difference in speed and BC, But ballistic calculator shows only difference in about .2-.3' vs. 3'-4' I see.
BTW I was using estimated BC - can't find correct ones on Win site.

Regarding Barrel Time and Recoil - I was expecting 174Gr bullet to POI lower than 180Gr. I'm shooting Savage 12FVSS. 26' heavy free-floating and button-rifled barrel. What can be wrong with the load? I’m still scratching my head....

 

[Mikem]

In my 308, and for target shooting, I normally shoot 168g SMKs and I have the rifle and scope zeroed for 100 yds. If I switch to 155g Palmas at about the same speed or a little faster, they will all group tight, but will be 2 to 3' off at 2:00 o clock. I guess that's just the way the critters work. And what the knobs on the scope are for.
I have found that almost any change to a load, and the POI will change. Now if the load is in its 'sweet spot', a slight change to the power load may only make a slight difference in elevation.

 

[Travelor]

Barrels move,whip) with the firing of the cartridge - in a circular motion as looked at from the muzzle. Different loads cause the bullet to exit the barrel at different point in the barrel movement.

That is why the POI is different, not the barrel time. If it were barrel time, the slower load, no matter what bullet weight, would always shoot higher,until gravity takes over) as the barrel would be shifting upward as a result of recoil before the bullet left the barrel.

If you have loaded the very light/heavy bullets into a pistol or changed the load to change velocity, you would notice the change in POI due to the barrel time vs barrel shifting upward caused by recoil. When I started shooting revolvers in IDPA this became a real factor when gettng the powder factor to be jsut above legal.

BTW - this knowledge will help you get a fixed sight pistol to shoot to point of aim,at least vertically) - by changing bullet weights and or velocities.

 

[fdshuster]

strobes: I agree with Travelor. My 308 loads for a Tikka barreled 1-10 twist, all 'things' being equal, 45 grs. Varget, Fed 210, touching,Lapua brass, etc), but one 'batch' with the Sierra 155 BTHP# 2155, and the other with the 155 A-Max#30312, will have significantly different group locations on the target: I must make small adjustments in both windage & elevation.

 

[strobes]

called Savage and found that proper torgue for my 12FVSS is 25 inch pound for screws for the bases. Will see if this may change the barrel flex. It looks like I'm dealing with Downward Slope.
Any recommendation how to change it to Upward Slope?
http://www.varmintal.com/aeste.htm

 

[strobes]

I was at the range yesterday with a 6 Dasher. Twenty-five rounds with Berger 105 VLD's and from 31.8 to 32.8 grains of RL-15 jammed 0.020'. The point of impact changed for each powder charge but all were one hole groups and the POI varied less than an inch from one charge to another.

Also had ten rounds of 31.8 grains that were jumped 0.010'. POI was over three inches to the left but vertically the same as the same load jammed. Again, both five shot groups were in one hole.

The barrel, even the HV contour Bartlein on mine,'flops around' a lot. You might want to read Varmint Al's explaination of this on his site:

http://www.varmintal.com/amode.htm

If you want to get really serious do an Internet search on Optimum Barrel Time. If you're willing to invest the effort and have QuickLoad to work with it's quite valuable. TechShooter,I think) has several posts on it here as well as a website about developing a load for a 6 Dasher:

http://www.the-long-family.com/6_dasher.htm

Really enjoyed reading 6_dasher OBT article and downloaded the OBT calculator.
And want to verify is barrel length expected to be used in calculator that is listed in manufacturer’s specification?,for ex. 26” for 12FS) Or as it has been mentioned in the article calculated from the face of the bolt?

 

[scout1]

strobes,

In order to benefit from Longs OBT research you must incorperate the Quickload program callibrated to your specific lot of powder. He has included instructions on how this is done on his site. Its really kinda cool but havent had much luck with it mainly due to a crappy chronograph.

 

[strobes]

strobes,

In order to benefit from Longs OBT research you must incorperate the Quickload program callibrated to your specific lot of powder. He has included instructions on how this is done on his site. Its really kinda cool but havent had much luck with it mainly due to a crappy chronograph.

Where are you? I'm in Plano, TX and have MKIV XP. Welcome to use it.

Still the same question remaining what barrel length to use in the calculator? I have enough empirical data to compare calculated OBT with field observations.

 

[scout1]

strobes,

apreciate the offer, but I'm in Fla. I just use the provided spread sheet or as Long calls it the 'comprehensive list' of barrel times based off of barrel length. I work up a safe load using one of the nodes listed for my barrel length. I then chronograph it and callibrate the Q.L. burn rate to my specific powder to match the program predicted speeds to my realworld chronographed results.
I use the total barrel length as opposed to bullet travel length. When you buy a barrel, its finished length is measured from end to end including the threads.

 

[mikeincool]

Hey Strobes, I'm in Fort Worth. Where are you getting your primers in the metroplex? Also, where do you shot?
Thanks, new to this sight.

 

[strobes]

I shoot in garland public range,
Internet.

 

[MikeyG]

Just looking at physics and not taking into account rifle characteristics - If you fire a bullet horizontally and drop a bullet from the same height as the rifle at the same time then they will both hit the ground at the same time. Bullet drop between two different weight bullets fired at the same elevation will happen at the same speed regardless of the bullet weight,ignoring friction)

The lighter bullet goes faster so reaches the target faster and has less time to drop.

EDIT:
Just crunched the numbers. Assuming there is no friction,otherwise the maths is too hard) the 147 grain load would drop 2.20 inches and the 180 grain load would drop 2.53 inches in 100 yards. This gives a 0.33 inch change in impact point.

As friction does occur and makes the bullets slow down actual change in impact point would be different, but not to the point of 3-4 inches at 100 yards.

Looking at these results the barrel whip, recoil, etc explanations seem to be more realistic than bullet speed differences.