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Discussion Starter · #1 · (Edited)
Goals of the shoot: To get the numbers for my custom Leupold Mark 4 MR/T BDC dial.



Chronograph Results
Smith Ent. M14 Scout 18.0” NM medium wt.
Crazy Horse barrel 1:10 twist​

Fed GM308M SMK 168grn BTHP:
1. 2670 fps
2. 2609
3. 2692
4. 2632
5. 2631
6. 2606
7. 2630
8. 2591
9. 2551
10. 2535

1st String:
Sum: 26147
Mean Average: 2614.7 fps
Standard Deviation: 45.73

2nd String:
Ten entries:
Sum: 25984
Mean Average: 2598.4 fps
Standard Deviation: 11.12


Fed AEM1A 168grn OTM
1. 2549
2. 2566
3. 2516
4. 2559
5. 2550
6. 2555
7. 2555
8. 2554
9. 2573
10. 2550

Sum: 25627
Mean Average: 2562.7 fps
Standard Deviation: 38.9

Fed GM SGK P308C Softpoint 165grn hunting :
1. 2577
2. 2549
3. 2572
4. 2554
5. 2580
6. 2563
7. 2547
8. 2555
9. 2562
10. 2574

Sum: 25633
Mean Average: 2563.3 fps
Standard Deviation: 11.33

AB M118 LR 175grn BTHP
1. 2472
2. 2593
3. 2593
4. 2588
5. 2554
6. 2581
7. 2588
8. 2579
9. 2608
10. 2596

1st String:
Sum: 25752
Mean Average: 2575.2 fps
Standard Deviation: 36.7

2nd String:
Ten entries”
Sum: 25465
Mean Average: 2546.5 fps
Standard Deviation: 27.023

Playing with the numbers:

Cold barrel 1st shot sum, all groups:
(Note: 2 taken from Fed SGK hunting 165grn and two from AEM1A OTM)

8 entries:
Mean Average: 2572.5 fps
Standard Deviation: 52.66 fps


Mean Average Total, all groups:

2576.33 fps
 

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Discussion Starter · #2 · (Edited)
Called in the numbers today to Leupold's Custom shop for my BDC dial:

Cartridge Type: 308
Bullet: Sierra Match King
Bullet Wt: 168 grn
Ballistic Coe: .460
Muzzle Velocity: 2576
Average Altitude: 1500 ft.
Average Temp: 50 F
Pressure: 30.00

They are going to etch the top of the dial: 308 168 grn M14 Scout so I don't mix it up with the old one. Said it will take four weeks. $60.00 Really stoked to try it out!
 

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Outta curiosity, was the bore cleaned before the first shot? Do any fouling rounds?

I plan to do some chronying and make ballistic card as soon as spring comes, but it'll have a 22" barrel with handloads.

Thanks for posting!
 

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Called in the numbers today to Leupold's Custom shop for my BDC dial:

Cartridge Type: 308
Bullet: Sierra Match King
Bullet Wt: 168 grn
Ballistic Coe: .460
Muzzle Velocity: 2576
Average Altitude: 1500 ft.
Average Temp: 50 F
Pressure: 30.00

They are going to etch the top of the dial: 308 168 grn M14 Scout so I don't mix it up with the old one. Said it will take four weeks. $60.00 Really stoked to try it out!
Where did that BC number come from? Comparing to the BCs on Sierra's website it sounds a little high. Depending how far you want to use the BDC, you might want to use a number more like .435 which will split the difference out past 600 yards or so. The Game King 165 SBT will come out more like .410-.415 in case you want to split the difference there again.

You might get a more accurate BDC cam if you ran the trajectories on a good ballistic software using all of Sierra's BCs as published (The SMK 168 has four BCs for different velocities) then give the results in inches drop at ranges to Leupold for the cam.

EDIT: I just ran the Sierra 168 SMK in the Sierra Infinity ballistic software using your data with a BC of .460 all the way out, then ran it again using Sierra's BCs (lower than .460). Your cam would be about 2" high by 600 yards, and about 8" high by 800 yards. That's not a lot of difference, especially not that far out.

.308 dia. 168 gr. HPBT MatchKing
.462 @ 2600 fps and above
.447 between 2600 and 2100 fps
.424 between 2100 and 1600 fps
.405 @ 1600 fps and below

MCORPS1
 

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Discussion Starter · #5 · (Edited)
Bricktop, from my understanding fouling rounds are related to accuracy. I was trying to get the FPS of my bullet and not checking accuracy, although I did shoot 60 rounds through a 1 inch hole ten feet away during the test. I always keep a light coat of oil in the barrel because its unlined and never wipe it dry before shooting. My rifles are meticulously cleaned.
 

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Discussion Starter · #6 · (Edited)
2336 Devil dog,

"Your cam would be about 2" high by 600 yards, and about 8" high by 800 yards. That's not a lot of difference, especially not that far out."

Can you interpret this? Do you mean my point of impact would be 2" low at 600 yrds based on the high cam? Or high? How will this affect my point of impact? I'm not a police marksman but a military marksman; any hit anywhere on the body is a good hit. I just want a quick down and dirty BDC dial to get me hits in a fire fight.

I got that BC from Sierra's website and made a few adjustments based on my load choices. I'm only shooting to 700 yards. Would you mind plugging in my numbers to that distance, 100, 200, 300 ect to 700 yards, and tell me what your program says? Leupold asked me what zero I wanted my dial at. I told them 100 yards.

As far as the 165 grn SGK hunting load, its BC is close enough up to 200 yards with the 168 grn SMK to not make any difference. I doubt I'll ever make a shot on a deer in Missouri over 200yrds. Very wooded and hilly.

Thanks for your help.
 

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Sure, in other words, you are calling your BC a bit on the high side, so when Leupold makes the BDC cam they will set it up for a flatter trajectory than you will actually see. Or, the bullet will drop a bit more than the cam adjusts for.

It's not a big difference and maybe less than most people can see in the real world, but still if you're gonna go custom why not go perfect... GI6
 

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Discussion Starter · #8 · (Edited)
Thanks for showing that. It helped a lot and actually confirms that my figures are correct based on perameters that I did not fully explain. Since the majority of my shooting will be with FGMM SMK 168 grn, which chrono'ed out of my rifle on a cold barrel an average of 2614 fps, I chose .460 BC, backing it off .2 for the slower rounds that I tested here and shoot but not as much. So the 2576 fps value is the medium for all bullets I use, and the higher .460 BC counters it somewhat and streamlines my faster bullets.

According to the chart, if I'm reading it right, at 700 yards, I'm low by 3.01" with the slower bullets, and theoretically spot on with the faster bullets. I'm going to test shoot the dial out to 600 yards and check it's zero every 100 yard increment.

Unless you see a major flaw in my logic, I feel this is the best set of numbers. But I am open to suggestions if you run some different numbers and come up with a better formula. I can still call Leupold and change it.
 

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then ran it again using Sierra's BCs (lower than .460). Your cam would be about 2" high by 600 yards, and about 8" high by 800 yards. That's not a lot of difference, especially not that far out.

.308 dia. 168 gr. HPBT MatchKing
.462 @ 2600 fps and above
.447 between 2600 and 2100 fps
.424 between 2100 and 1600 fps
.405 @ 1600 fps and below
Sorry to interrupt but I'm contemplating the same scope, for a similar rifle, for all the same reasons.

Are you suggesting the numbers I highlighted, based on his chrono results, instead of the
.462?
 

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Discussion Starter · #10 ·
Led, I don't think he was suggesting, I think he was showing the differences so I can make an informed decision.

By the way, this MR/T scope is a great tool and in my opinion almost a perfect match for the M14 marksman rifle. It's not big or long or heavy, only 16oz, and very tough. The fact that you can customize it to match your load and rifle's shooting habits is way cool.
 

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I see where you are going with this. By over-fudging the BC and under-fudging the MV for the GMM you get a pretty close match to the trajectory. But, by over-fudging the BC AND over-fudging the MV for the Fed 168 OTM you are pretty far off.

Here's another approach. Get the best match possible to the trajectory of the ammo you want to shoot the most (FGMM 168), then make a simple scope adjustment to match other ammo.

Take a look at these charts. The "BDC Cam" chart assumes a MV of 2600 and a BC of .434 which is a best fit if you can only quote one BC to Leupold. The second chart is FGMM 168 at 2600 fps using Sierra's BCs. The trajectory matches within less than an inch all the way out to 700. The third chart is Fed 168 M1A OTM at 2560, zeroed 1/2" high at 100 (just add a couple of clicks to the scope when you shoot this ammo.) The added 1/2 MOA raises the trajectory of the slower bullet to also be within +/- 1 inch all the way out to 700 yards.

A BDC cam with those specs will be so close that it will be more accurate than your eyeball, or the rifle, or the ammo, and you would have to start worrying about temperature and humidity and whatnot to get any more accurate.

BTW, M118LR type with Sierra 175s will hit high because the BC is so much higher than the 168. Unless you just get lucky and your rifle tends to throw the heavier bullets low... but you can match the 175 to that cam pretty close by the same trick of just cranking on an elevation adjustment to the scope when you want to shoot 175s.

What we don't know is what software Leupold uses to take your stated MV and BC and convert them into MOA for the clicks on your custom BDC cam. I have been using Sierra's program for years, and when you are shooting Sierra bullets the numbers are dead on.

Hope this helps. MCORPS1
 

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According to the chart, if I'm reading it right, at 700 yards, I'm low by 3.01" with the slower bullets, and theoretically spot on with the faster bullets.

Unless you see a major flaw in my logic, I feel this is the best set of numbers.
The only flaw in logic that I can see with your plan, is the muzzle flip variable. If you are low with the slower bullets, you will need to figure in the flip and/or rise of the muzzle for faster and lighter bullets. A faster bullet will clear the muzzle quicker than a slower one, and not be affected by flip as much, therefore hitting lower than a slower bullet, by a small amount though. At that distance, maybe 1 1/2 "-2".......The same is true for lighter bullets because of their additional speed, they will always hit lower than slower or heavier bullets.
 

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Bookmarking this thread.
Gonna break the chrono back out when the weather breaks or hopefully sooner if the mud just dries up.
BlackArrow will likely have concrete results, before I even have scope in hand to order a dial, but I just don't think I can quite hit that majic 2600FPS mark. The OP's 2572 FPS is more in line, to date anyway.
 

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The only flaw in logic that I can see with your plan, is the muzzle flip variable. If you are low with the slower bullets, you will need to figure in the flip and/or rise of the muzzle for faster and lighter bullets. A faster bullet will clear the muzzle quicker than a slower one, and not be affected by flip as much, therefore hitting lower than a slower bullet, by a small amount though. At that distance, maybe 1 1/2 "-2".......The same is true for lighter bullets because of their additional speed, they will always hit lower than slower or heavier bullets.
This is true... We need to remember that "zero" is not the same thing as "trajectory." Trajectory is very predictable, a certain bullet traveling a certain speed through a certain kind of air will have a certain trajectory and it can be predicted very accurately.

"Zero" is adjusting your sights so that they match that trajectory, at least at one distance. A BDC cam is a fancy gadget for zero that hopefully will match the trajectory at any distance, after you get a basic zero at say 100 yds.

Unfortunately, zero is not very predictable and it can vary a lot between different rifles and different ammo. It is possible to have a rifle that will shoot the FGMM at 2600 or the Fed OTM at 2560 both into the same hole at 100, or another rifle might throw those two rounds 4 MOA apart. It can't be predicted, it has to be tested.

The difference in the trajectories of the two bullets can be predicted, the difference in zeros of the two bullets can't be predicted.

That's part of why I recommend setting up the BDC cam to closely match the ammo you will shoot most. That will be your primary zero. If other types of ammo that you shoot only once in a while, like the Fed M1A OTM, end up shooting to a different zero you can just keep notes on that so you can reset the scope when you change ammo types. In the example I gave, when changing from your primary ammo (FGMM at 2600) to your secondary ammo (Fed M1a OTM at 2560) you would just make the change to the scope zero based on your notes, plus add 1/2 MOA of extra elevation that would make the trajectory of the BDC cam closely match the trajectory of the bullet.
 

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Discussion Starter · #15 · (Edited)
Ripsaw, I wonder if a Vortex flash hider will make any difference in the muzzleflip. Since there is no ring or edge at the end like a traditional bird cage supressor, maybe gas expulsion will be more uniformed regardless of bullet speed.

Edit: I see what you're saying after reading it again. You're talking about the kick of the rifle after firing and the faster bullet will exit before the flip. OK--that's something to factor in.
 

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Discussion Starter · #16 · (Edited)
2336USMC,

"Get the best match possible to the trajectory of the ammo you want to shoot the most (FGMM 168), then make a simple scope adjustment to match other ammo."

I think I'm going to take your advice on this. I'm really amazed I'm getting 2600 FPS or higher out of a 18.5" barrel. The OTM costs half as much as the FGMM and there is a reason.

So the BC to .462 and the fps 2600, then record adjustments for the other loads. Sounds like a plan........I just got off the phone with Leupold and have requested the changes.

Thanks so much for your help!

Now I'm wondering if the original dial is already calibrated to the above specs because it's so generic?
 

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So the BC to .462 and the fps 2600
Maybe still a little smoke in the air...GI6

You're not going to see a BC of .462.

The reason Sierra lists 3, 4, or even 5 BCs for each bullet is the BC changes as the bullet flies thru the air and slows down. The way the air and shock waves and whatever else flows changes with speed, so the BC changes. For some bullets the BC might go up as it slows down, for other bullets the BC goes down as it slows down. For the 168 SMK it BC goes down.

.308 dia. 168 gr. HPBT MatchKing
.462 @ 2600 fps and above
.447 between 2600 and 2100 fps
.424 between 2100 and 1600 fps
.405 @ 1600 fps and below

.462 only applies while the bullet is going faster than 2600 fps... since that's about the speed you are getting at the muzzle and it's all downhill from there you can toss out that .462 number completely.

From the muzzle out to about 250 yards the BC will be .447. From 250 yards out to about 550 yards the BC will be .424. From 550 on out the BC will be .405.

Since Leupold didn't ask you for 3 BCs with velocity envelopes, you need to split the difference and pick one number that gives a good approximation. Right around .434 or .435 gives a trajectory that is a very close match. .462 will cause all of your ammo to shoot low (cam will think the bullets fly flatter than they really do) and you will be holding over for everything.

HTH MCORPS1
 

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You cleared me right up with that one 2336USMC.
Feel like I just stole dinner off a couple of stranger's plates but the knowledge is appreciated, the charts I saved to my computer, and I was going to have to ask it sooner or later.
Called them last week to figure out what I needed to give them before ordering the scope(as it turns out they will ship the dial later)....so thanks again guys.

Will also be picking up that software.
 

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Discussion Starter · #19 ·
Oh well, I'll just make a few adjustments to the scope or hold the rifle loose for more flip or shoot at a higher altitude---ha ha!

Thanks
 

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Good Lord ,As I was There Jotting those numbers down,I am So lost here it is not Funny.
It is interesting Just the Same Lost LOL
 
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