Questionable Velocity Indications

[Pirate69]

My appologies for bring up a J-bullet load but I need some experienced advise.

I have a hunting load for my 24 inch barrelled 300 Win Mag that is shooting less than 1 MOA. Really. I checked the drop at 100 yards, 300 yards and 400 yards by zeroing at 100 yards and moving the target to the other two distances. All measured distances. Measured drop from 100 yards zero POI to 300 yards POI was 9.25" and measured drop from 100 yards zero POI to 400 yards POI was 20.5". Knowing the air temperature and the barometric pressure, I tried to back the drop results into a ballistic calculator and determine the muzzle velocity. Indicated muzzle velocity is approximately 3180 fps. This muzzle velocity and the known ambient conditions gives me a 300 yard drop of 9.25" and a 400 yard drop of 21.1". Not a bad agreement. The fired primers do no show any pressure indications and the bolt lifts without any indication of resistance. I know Hornady has a 180 grain load that is advertized at 3130 fps but my indicated velocity seems high. Trying to figure out if I am overlooking something. Specifics for my load and results are:

Bullet: 180 grain Sierra Spitzer Boattail
BC: greater 2700 fps; 0.501---greater 1700 fps but less 2700 fps; 0.506---1700 fps or less; 0.505
Load: 74.0 grains IMR 4831; magnum primer
COL: 3.496"
Case Ln: 2.611"
Bullet Ln: 1.28"
Sight Above Bore: 1.46" (measured to confirm 1.5" scope mount)

Air Temp: 61 F
Bar. Press.: 30.13
RH: 40%

Any and all comments/suggestions will be greatly appreciated.

[Iron Whittler]

Sounds like you found the sweet load for this particular rifle. It is accurate, repeatable, and from physical indications, a good load for hunting. Now, all that is needed is game in the crosshairs= meat on the table and in the freezer. Good hunting. NOTE-- I do not know if you are aware of a common issue with belted mag cases. The area just above the belt does not resize with usual sizing dies. After 3 or 4 loadings, it is common to have cases that are hard to chamber due to swelling just above the belt. There is a special collet die made just for sizing the area above the belt. It can be used on most of the Win Mag belted case cals. Iron Whittler :castmine:

[flounderman]

How high is your scope mounted above the bore. It has been my experience using as low a mount as I can use, that a rifle sighted dead on at 100 yards falls off the table after that. I sight 1 1/2 inches high at 100 and the bullet is higher some where towards 200 yards and then recrosses the line of sight further out. You are going to get a different curve with a high mount. The bullet starts dropping as soon as it leaves the muzzle. Your line of sight is a straight line. The bullet has to be angled to come up to or above the line of sight. The higher the line of sight, the more the bullet has to be angled. I'm guessing you have a high scope mount. Seating depth, whether the bullet is allowed to move before engaging the rifling or is seated into the rifling will change pressure, the resistance the barrel has from bore diameter and smoothness makes a difference. Most factory results are measured with a 26 inch pressure barrel unless they state otherwise. The only way to know what you are getting is with a chronograph. I don't live that far from you and I can loan you one with the agreement if you shoot it, you replace it. I was shooting a case full of wc860 and 165 grain bullet in a 300 and was totally disappointed when I found out how little velocity I was getting when I got a chronograph. I'm shooting 77 grains of wc 852 behind a 165 grain now according to my records. Tom oswleys sheet that came with the powder says 72 grains and most of his data was about maximum. My 852 in some calibers acts about like 4831. I didn't see any chronograph results for this load.

[Pirate69]

Flounderman,
Thanks for the offer to loan a chronograph. I think I will be able to borrow one locally a little later. The center of sight above the bore is 1.46" (1.5"). I am happy with the rifle's performance and know how to hold for various ranges. I am just curious about the indicated velocity.

I have taken the indicated 3180 fps muzzle velocity and plugged all the data into another program, the JBM Trajectory Calculator. The JBM predicts a drop of 9.4" at 300 yards and a drop of 21.5" at 400 yards. Actual drop data indicated a 300 yard drop of 9.25" and a 400 yard drop of 21.1". Everything is pointing to an actual velocity of around 3180 fps. Got to get some time and get my hands on a chrony.

Would be interesting to know what QuickLoad says the velocity would be at those load charactertics.

[dtknowles]

I don't have a Sierra manual but my 4th ed. Hornady manual says max is 69.0 gr. of IMR4831 with a velocity of 2900 fps with their BTSP so you could be getting 3180 fps with 74.0 gr. The IMR powder data on the Hodgden website does list a max of 74.0 gr. with a Speer 180 gr. SPR MT-SP. It also lists that as a compressed load and a velocity of 3025 and a pressure of 63,200. Where did you get your load data. Both these data points are from 24 inch barrels.

What size groups were you getting with the other loads as you worked up to this potentially over max. load. If the groups were good I would drop back a bit on the powder charge, you tested on a mild day. I would want some margin for a hot day or warm ammo. I think you are out there on the ragged edge. Not Kaboom type ragged edge but pierced primer or stuck bolt type problem. Definitely in the regime where you would need to drop back and work back up if you change anything, powder lot, primer lot, bullet lot, new cases, overall length, brass length. Just increase in neck tension from multiple resizing of the cases could be the straw that breaks the camels back.

Yes, there is margin in loading data but it is there for good reasons and the margins are much smaller for modern calibers.

Tim

[Scharfschuetze]

You selected a very good long range bullet.

Let me muddy the waters a bit with the data from a different ballistics program.

I ran the 180 grain Sierra spitzer boattail bullet at your estimated velocity of 3,180fps with a 100 yard zero and your sight 1.5" above the bore on a standard atmospheric day through my "Ballistic AE" app on my iPhone. It's a pretty good app and has made good predictions in the past.

Your results:

100 Yards = 0
300 Yards =-9.25"
400 Yards =-20.5"

Ballistic AE at 3,180 fps:

100 Yards = 0
200 Yards = -1.9
300 Yards = -7.7"
400 Yards = -17.5"

I ran a corrected velocity of 3,000 fps with the remaining data the same and came up with results very close to your "on paper" results.

100 Yards = 0
200 Yards = -2.2"
300 Yards = -9.5"
400 yards =-21.0"

[Larry Gibson]

Lyman's #49 shows 73 gr IMR 4831 as a max load (63,000 psi) load under the Sierra 180 MK at 3035 fps (24" barrel also).


Thus, at 74 gr you are at a max load. Probably ok considering the differences between your barrel and a pressure barrel.

I've also measured enough BCs, velocities and down range POIs to know that any guess at velocity only gets you close; +/- 150 fps.

Your backwards figuring for the velocity is a good SWAG but is probably a bit optimistic. You are probably closer to the 3000 - 3100 fps range. Be interesting to chronograph the load in your rifle though.

Larry Gibson

[Pirate69]

Lyman's #49 shows 73 gr IMR 4831 as a max load (63,000 psi) load under the Sierra 180 MK at 3035 fps (24" barrel also).


Thus, at 74 gr you are at a max load. Probably ok considering the differences between your barrel and a pressure barrel.

I've also measured enough BCs, velocities and down range POIs to know that any guess at velocity only gets you close; +/- 150 fps.

Your backwards figuring for the velocity is a good SWAG but is probably a bit optimistic. You are probably closer to the 3000 - 3100 fps range. Be interesting to chronograph the load in your rifle though.

Larry Gibson

I plan on measuring the velocity as soon as possible.

[Pirate69]

I plan on measuring the velocity as soon as possible.

I was thinking about Larry's comment on this being a SWAG and thought I would look at how big a SWAG this exercise could be. I finally figured out one important fact that I was ignoring. If a rifle is shooting MOA or less, at 300 yards, the round could hit anywhere in a 3" circle; 4" circle at 400 yards. For sake of augument, I assume the POI at each range was the center of the circle. I then looked at what the projected velocities, by this method, would be at the top and bottom of the circles (plus or minus 1.5" and plus or minus 2.0"). At the 300 yard target, the velocity range was 3,000 fps to 3,400. At the 400 yard target, the velocity range was 3,075 fps to 3200 fps.

Looks like this is a poor method of determining a velocity and will not threaten the sale of any chronographs. This agrees very well with Larry's prediction that the method is in the +/- range of 150 fps. I do not own QuickLoad but I would not be surprised if Quickload gave a better estimate, compared to a chronograph, than this method does. Oh well, it was fun doing the exercise.

[Scharfschuetze]

You have a very valid point if you measured from just one impact at each range.

I thought that you had fired groups and then measured from the center of each group for your downrange trajectory measurements.

It will be interesting to find out what your load's actual velocity is.

The only magnum rifle I ever owned was a 7mm Remington. Those were pre chronograph days and after I worked up what I thought was a maximum and accurate load (175 grain spitzer at 2,900 by the book) at 100 yards, I tried it at 200, 300 and 500 yards. Using Hornady bullets and of course their current manual of the time, I was quite surprised at how much drop I actually had compared to what the book predicted. It was still a great game load, and with my revised and true trajectory tables I had good success with it in Colorado and Wyoming. Never shot a thing with it over 120 yards or so.