.308 what gained by 30" bbl
- Thread starter onegun2
- Start date
Paying extra if it has the face painted on it.
Jimro, I believe your data from JBM is what it calculates. But I don't think it's reality.
From Sierra Bullets' software with a 30 caliber 175 HPMK leaving at 2600 fps with 59 deg, 78% RH, and 29.92 pressure....
1214 fps at 1000 yards.
Mach 1 = 1116.15 fps with above atmospheric conditions.
Bullet's moving at Mach 1.08767 at 1000 yards.
That aside, depending on whose software you use, there's typically a 3 to 5 percent spread in results. JBM's software only allows 1 BC number. What BC did you use for the 175? Sierra's proved their bullets have different BC's for different velocity bands; if that's not used in the software, its calculations won't be accurate. Their G1 BC numbers for the 175:
.505 @ 2800 fps and above
.496 between 1800 and 2800 fps
.485 @ 1800 fps and below
I used JBM's software for a 175 MK leaving at 2600 fps with a G7 BC of .327 and it went 1407.3 fps at 1000 yards; Mach 1.260.
Having measured Sierra's bullet velocities with a couple of their 30 caliber match ones then calculated down range drop at 300 and 800 yard drops with their software, converted drop numbers to sight settings put zeros within 1/4 MOA of actual with actual range atmospheric conditions used. Pretty much convinces me Sierra's software is excellent for their bullets.
From Sierra Bullets' software with a 30 caliber 175 HPMK leaving at 2600 fps with 59 deg, 78% RH, and 29.92 pressure....
1214 fps at 1000 yards.
Mach 1 = 1116.15 fps with above atmospheric conditions.
Bullet's moving at Mach 1.08767 at 1000 yards.
That aside, depending on whose software you use, there's typically a 3 to 5 percent spread in results. JBM's software only allows 1 BC number. What BC did you use for the 175? Sierra's proved their bullets have different BC's for different velocity bands; if that's not used in the software, its calculations won't be accurate. Their G1 BC numbers for the 175:
.505 @ 2800 fps and above
.496 between 1800 and 2800 fps
.485 @ 1800 fps and below
I used JBM's software for a 175 MK leaving at 2600 fps with a G7 BC of .327 and it went 1407.3 fps at 1000 yards; Mach 1.260.
Having measured Sierra's bullet velocities with a couple of their 30 caliber match ones then calculated down range drop at 300 and 800 yard drops with their software, converted drop numbers to sight settings put zeros within 1/4 MOA of actual with actual range atmospheric conditions used. Pretty much convinces me Sierra's software is excellent for their bullets.
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Brian Pfleuger
Moderator Emeritus
I have found every single ballistics and stability calculator to be pessimistic. Bullets that they say will not be stable often are and trajectory is better than expected. The ballistics usually works better if it's set to G7 instead of G1, for any bullets. Also, stability calculators often don't account for the plastic tip. JBM now does.
I bet the JBM would be more accurate set to G7 for that bullet.
I bet the JBM would be more accurate set to G7 for that bullet.
Brian, I agree with your assesment of ballistic software. It's often a "rubber ruler" that gets stretched to the developer's and user's whims.
I did observe one test between Berger's 30 caliber 185-gr. VLD bullet to Sierra's 190-gr. HPMK shot from a .308 rifle. Two chrono screen sets were used; one at 5 yards and the other at 998 yards from the muzzle. Bullet velocities at the near screens were around 2580 fps. Velocity for a given bullet at the far screens showed the Berger VLD to loose about 45 fps more than the Sierra HPMK did. Berger's VLD's supposed to have a G7 BC of .281 and Sierra's HPMK's G7's listed at .270. That actual test says the Berger bullet has a lower BC.
I did observe one test between Berger's 30 caliber 185-gr. VLD bullet to Sierra's 190-gr. HPMK shot from a .308 rifle. Two chrono screen sets were used; one at 5 yards and the other at 998 yards from the muzzle. Bullet velocities at the near screens were around 2580 fps. Velocity for a given bullet at the far screens showed the Berger VLD to loose about 45 fps more than the Sierra HPMK did. Berger's VLD's supposed to have a G7 BC of .281 and Sierra's HPMK's G7's listed at .270. That actual test says the Berger bullet has a lower BC.
Bart B.
I used Litz's G7 data for the 175 SMK, also used the same for the 155, except for the Scenar which I used the published data.
All of the shooting I've done with the 175smk has been from M118LR at altitudes slightly (Joint Base Lewis McChord) to significantly above sea level (Yakima Training Center). It doesn't take much altitude (reduction in pressure) to make the SMK stay supersonic past 1k.
Based on the data we gained using M24s, going from 80 feet to 2000 feet extended the maximum range by 300 yards, from 1100 to 1400.
Still, based on what I observed I'll maintain that atmospherics can keep the SMK from being supersonic at 1k. The published velocity for M118LR is 2580 fps at 78 feet from the muzzle, which corresponds to an at muzzle velocity of 2618 fps, but even at sea level the M118LR stayed accurate through the transition zone.
Jimro
I used Litz's G7 data for the 175 SMK, also used the same for the 155, except for the Scenar which I used the published data.
All of the shooting I've done with the 175smk has been from M118LR at altitudes slightly (Joint Base Lewis McChord) to significantly above sea level (Yakima Training Center). It doesn't take much altitude (reduction in pressure) to make the SMK stay supersonic past 1k.
Based on the data we gained using M24s, going from 80 feet to 2000 feet extended the maximum range by 300 yards, from 1100 to 1400.
Still, based on what I observed I'll maintain that atmospherics can keep the SMK from being supersonic at 1k. The published velocity for M118LR is 2580 fps at 78 feet from the muzzle, which corresponds to an at muzzle velocity of 2618 fps, but even at sea level the M118LR stayed accurate through the transition zone.
Jimro
A Sierra rep told me that their 175 MK's BC was the same as the military M2 FMJBT 173-gr. match bullet; or close enough to not be an issue, in their time of flight tests calclating BC's. That old machine gun bullet from the 1920's stayed supersonic through 1000 yards in half worn out M14 barrels in cool weather at the Nationals' 580 ft. altitude. I doubt it left the flashhider more than 2580 fps. Some folks claimed that bullet had the same BC as Sierra's 190 HPMK but that's stretching things a bit past reality.
Bart B.
2580 at the chrono for M118 would be 2618 at the muzzle. Considering a charge of 44gr of IMR4895 calculates to 2640 at the muzzle, I think that the bores would be have to be really worn out to be that slow with M72 style bullets.
But 580 ft elevation with standard temp and pressure for altitude with that muzzle velocity would give you a transition at 990 yards, which is inside the "fudge factor" for atmospherics, a hot day increasing velocity, etc.
At 80 feet altitude at Ft. Lewis, on a high pressure high humidity cold day, it works the other way.
Jimro
2580 at the chrono for M118 would be 2618 at the muzzle. Considering a charge of 44gr of IMR4895 calculates to 2640 at the muzzle, I think that the bores would be have to be really worn out to be that slow with M72 style bullets.
But 580 ft elevation with standard temp and pressure for altitude with that muzzle velocity would give you a transition at 990 yards, which is inside the "fudge factor" for atmospherics, a hot day increasing velocity, etc.
At 80 feet altitude at Ft. Lewis, on a high pressure high humidity cold day, it works the other way.
Jimro
Bart B.
You are correct, the charge for M118 was 42 gr IMR4895, only after WC846 got put into the mix that the spec was changed to 44 gr of either powder, but from what I can tell that was a very short period in the 1980s and that technical reference document was out of date back in the 90s when I first got ahold of it.
Must have been some hot non-cannister grade IMR4895 though, 42gr's works out to about 2520 fps at the muzzle with cannister grade powder, which is below the 2580 +/- 30 fps specified for M118.
Jimro
You are correct, the charge for M118 was 42 gr IMR4895, only after WC846 got put into the mix that the spec was changed to 44 gr of either powder, but from what I can tell that was a very short period in the 1980s and that technical reference document was out of date back in the 90s when I first got ahold of it.
Must have been some hot non-cannister grade IMR4895 though, 42gr's works out to about 2520 fps at the muzzle with cannister grade powder, which is below the 2580 +/- 30 fps specified for M118.
Jimro
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