168 grn 308 OTM Ballistic Coefficient

OfficerJohnson10 · November 12, 2010, 05:42 AM

Quick question....

Can anyone tell me what the ballistic coefficient is of the 168 grain OTM bullet that Federal uses in their American Eagle 7.62 x 51mm round?

After an extensive internet search I couldn't locate a coefficient specifically for this round and I just don't want to use the number provided for the Sierra Match King bullet Federal is using in their Gold Metal 168 grain 308 offering.

I'm assuming that the SMK and the OTM are different in some way because Federal took the time to name and list them separately so I'm hoping that one of you may have the info because I'm striking out short of calling Federal directly

Thanks in advance.

**Unclenick** · November 12, 2010, 10:03 AM

Short of calling Federal, the only way anyone else will know is if they measured it. I know one of the on-line retailers had some bulk OTM no-name bullets for sale that turned out to be made by Privi-Partizan. I think those were .223. They also turned out to have a lot of dimensional variation, which makes me doubt Federal would use something as carelessly made, but for white box, who knows? Calling Federal is about the only sure way to find out.

If you want to know how to measure BC's, read Bryan Litz's book. You can also use the JBM Calculators, but you need velocity at two ranges or transit time over a known distance to provide the drag effect for input into the calculator.

lmccrock · November 13, 2010, 06:57 PM

Quick check of the M1A OTM load data vs. 168gr Gold Medal Match ammo (which has the SMK) shows 0.1" different drop and 0.1" different wind drift at 500yds. Same muzzle velocity. So....probably not much difference in ballistic coefficient.

By the way, the M1A OTM ammo goes 2577fps from my 20" LR308 over my chrono. Just shot that today.

Lee

**Unclenick** · November 14, 2010, 08:13 PM

I had a senior moment and forgot Federal has those tables. Unfortunately they are all calculated from a constant G1 BC rather than measured. Federal uses a constant G1 BC of 0.462 for the 168 gr. MatchKing and a constant G1 BC of 0.460 for the OTM bullet. Sierra's own numbers for the MatchKing are 0.462 above 2600 fps, 0.447 above 2100 fps, 0.424 above 1600 fps, and .405 below that. Even these, however, are based on Sierra's lab conditions rather than outdoor firing, plus they are still G1 drag function based, which is not tracked by this bullet shape as well as the G7 drag function. So, I have Bryan Litz's measured G7 numbers taken at the actual distances at which the velocities drop, and that gives the closest real result.

As to drops, though, it's kind of a wash at just 500 yards. Only about 1.8" of elevation error and about 1/4" in 10mph crosswind. I think that's why Federal just uses the single G1 BC, figuring most people can't hold the sub-1/2 moa difference. If they were going to 1000 yards the error would become significant. At 600 the error has grown to 2.8", which merits an extra click of elevation on 1/2 minute sights. And the 1000 yard difference is moot since that 168 grain bullet tends to tumble when it gets into the transonic range. That starts at about 700 yards from my 24" barrels. The newer 175 grain SMK does not, if your looking for a heavy 1000 yard bullet.

Code:

Sierra 168 grain MatchKing
Yards  Federal Sierra	Litz
  0	2650	2650	2650
100	2460	2445	2447
200	2277	2267	2253
300	2103	2087	2065
400	1936	1906	1886
500	1778	1737	1713

So, without firing I don't really know the OTM bullet's BC's. But if I assume the real BC by scaling is .460:.462, but adjusting to track Litz's measured performance of the SMK BC's proportionally (and this is really a pile of assumptions at this point), then the following table for 200 yard zero is going to be close:

Code:

  Range    Velocity     Trajectory         TOF          Drift (10 mph 3:00 wind, no spin drift included)
(yards)     (fps)       (inches)          (sec)       (inches)
      0      2650          -1.50         0.0000         0.00
    100      2444           2.17         0.1179        -0.83
    200      2248           0.00         0.2458        -3.43
    300      2061          -9.06         0.3852        -8.04
    400      1882         -26.33         0.5375       -14.93
    500      1712         -53.44         0.7046       -24.42
    600      1548         -92.46         0.8889       -36.93

OfficerJohnson10 · November 15, 2010, 08:08 PM

Thanks to all for your replies
I think using the .460 BC will be sufficient for my needs since I am only looking to do some recreational longer range shooting at this time. When I get into more precision shooting I will more than likely be "rolling my own" and using a SMK and you'll be seeing my name much more frequently on this particular board but thats a whole different ball of wax

Once again thanks for the replies. The information was very helpful.

November 12, 2010, 05:42 AM	#1
OfficerJohnson10 Senior Member Join Date: February 6, 2007 Location: Eastern Pennsylvania Posts: 102	168 grn 308 OTM Ballistic Coefficient Quick question.... Can anyone tell me what the ballistic coefficient is of the 168 grain OTM bullet that Federal uses in their American Eagle 7.62 x 51mm round? After an extensive internet search I couldn't locate a coefficient specifically for this round and I just don't want to use the number provided for the Sierra Match King bullet Federal is using in their Gold Metal 168 grain 308 offering. I'm assuming that the SMK and the OTM are different in some way because Federal took the time to name and list them separately so I'm hoping that one of you may have the info because I'm striking out short of calling Federal directly Thanks in advance. __________________ "The day I've learned everything about policework is the day I retire." - Sgt Your goal in a gunfight should not be to simply survive...it should be to prevail.

November 12, 2010, 10:03 AM	#2
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,061	Short of calling Federal, the only way anyone else will know is if they measured it. I know one of the on-line retailers had some bulk OTM no-name bullets for sale that turned out to be made by Privi-Partizan. I think those were .223. They also turned out to have a lot of dimensional variation, which makes me doubt Federal would use something as carelessly made, but for white box, who knows? Calling Federal is about the only sure way to find out. If you want to know how to measure BC's, read Bryan Litz's book. You can also use the JBM Calculators, but you need velocity at two ranges or transit time over a known distance to provide the drag effect for input into the calculator. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

November 14, 2010, 08:13 PM	#4
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,061	I had a senior moment and forgot Federal has those tables. Unfortunately they are all calculated from a constant G1 BC rather than measured. Federal uses a constant G1 BC of 0.462 for the 168 gr. MatchKing and a constant G1 BC of 0.460 for the OTM bullet. Sierra's own numbers for the MatchKing are 0.462 above 2600 fps, 0.447 above 2100 fps, 0.424 above 1600 fps, and .405 below that. Even these, however, are based on Sierra's lab conditions rather than outdoor firing, plus they are still G1 drag function based, which is not tracked by this bullet shape as well as the G7 drag function. So, I have Bryan Litz's measured G7 numbers taken at the actual distances at which the velocities drop, and that gives the closest real result. As to drops, though, it's kind of a wash at just 500 yards. Only about 1.8" of elevation error and about 1/4" in 10mph crosswind. I think that's why Federal just uses the single G1 BC, figuring most people can't hold the sub-1/2 moa difference. If they were going to 1000 yards the error would become significant. At 600 the error has grown to 2.8", which merits an extra click of elevation on 1/2 minute sights. And the 1000 yard difference is moot since that 168 grain bullet tends to tumble when it gets into the transonic range. That starts at about 700 yards from my 24" barrels. The newer 175 grain SMK does not, if your looking for a heavy 1000 yard bullet. Code: Sierra 168 grain MatchKing Yards Federal Sierra Litz 0 2650 2650 2650 100 2460 2445 2447 200 2277 2267 2253 300 2103 2087 2065 400 1936 1906 1886 500 1778 1737 1713 So, without firing I don't really know the OTM bullet's BC's. But if I assume the real BC by scaling is .460:.462, but adjusting to track Litz's measured performance of the SMK BC's proportionally (and this is really a pile of assumptions at this point), then the following table for 200 yard zero is going to be close: Code: Range Velocity Trajectory TOF Drift (10 mph 3:00 wind, no spin drift included) (yards) (fps) (inches) (sec) (inches) 0 2650 -1.50 0.0000 0.00 100 2444 2.17 0.1179 -0.83 200 2248 0.00 0.2458 -3.43 300 2061 -9.06 0.3852 -8.04 400 1882 -26.33 0.5375 -14.93 500 1712 -53.44 0.7046 -24.42 600 1548 -92.46 0.8889 -36.93 __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

November 15, 2010, 08:08 PM	#5
OfficerJohnson10 Senior Member Join Date: February 6, 2007 Location: Eastern Pennsylvania Posts: 102	Thanks to all for your replies I think using the .460 BC will be sufficient for my needs since I am only looking to do some recreational longer range shooting at this time. When I get into more precision shooting I will more than likely be "rolling my own" and using a SMK and you'll be seeing my name much more frequently on this particular board but thats a whole different ball of wax Once again thanks for the replies. The information was very helpful. __________________ "The day I've learned everything about policework is the day I retire." - Sgt Your goal in a gunfight should not be to simply survive...it should be to prevail.

November 13, 2010, 06:57 PM	#3
lmccrock Senior Member Join Date: April 23, 2007 Location: Red Rock. TX Posts: 820	Quick check of the M1A OTM load data vs. 168gr Gold Medal Match ammo (which has the SMK) shows 0.1" different drop and 0.1" different wind drift at 500yds. Same muzzle velocity. So....probably not much difference in ballistic coefficient. By the way, the M1A OTM ammo goes 2577fps from my 20" LR308 over my chrono. Just shot that today. Lee