Author Topic: Ballistics question on 7.62x54R when shortening a barrel  (Read 4397 times)

0 Members and 1 Guest are viewing this topic.

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Ballistics question on 7.62x54R when shortening a barrel
« on: December 28, 2010, 12:25:54 PM »
Wasn't sure whether to post this here or in the ammo section so feel free to move it if needed mods. I put it here because I am asking this more on the rifle/barrel not on the ammo itself.

Okay, we are seeing the modified PSL builds on the market now that look like an AK on steroids. Nice looking rifles. The barrels on these are PSL barrels cut from 26.5" to 16.5" with the FH/MB reinstalled. I have a mixmaster M91/30 that I got from Longspring that was already sportered with the barrel cut down to about 22". Now my question is this in relation to both rifles as I am not a big ballistics guy.
Lets say, hypothetically, I want to make a 16"-18" carbine out of the MN and put a FH/MB on it giving it the same/similar attributes as the AK-FPK, as far as barrel length anyway. This is, of course, understanding that the MN is not chrome lined and the AK-FPK is. What is/will be changing in the ballistics of the round(say I am using standard 147gr LB in both) as concerns the dynamics of the ballistics, accuracy, recoil, and safety.
« Last Edit: December 28, 2010, 03:11:19 PM by Greatguns »

TheMechanic

  • SKS Marksman
  • ***
  • Posts: 156
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #1 on: December 28, 2010, 05:49:29 PM »
The link has good data on the affects of shortening a barrel. http://www.accuratereloading.com/223sb.html You can expect similar with 7.62X54R.

There was a reason that none of the military Mosin Nagants was less than 20.25" barrel length.

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #2 on: December 28, 2010, 07:35:55 PM »
Interesting, a drop of about 200 fps but little accuracy change. I did notice even with a velocity drop the rifle used seemed to be most accurate at 20". Thanks for the link.

Aye Kay

  • SKS Guru
  • **
  • Posts: 3054
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #3 on: December 28, 2010, 07:47:01 PM »
  It's still gonna hurt.  I'll take a short rifle over a long one anyday. 

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #4 on: December 28, 2010, 07:54:07 PM »
  It's still gonna hurt.  I'll take a short rifle over a long one anyday. 

Haha, what do you mean by that, hurt whatever I shoot at or hurt my shoulder?

Danjal

  • Not the droid you're looking for.
  • Life Member
  • SKS Guru
  • *****
  • Posts: 15794
  • Who I am /\. Things I say >
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #5 on: December 29, 2010, 01:21:30 AM »
Barrel langth has little to do with accuracy unless harmonics are in play. A 16" is just as accurate as a 30" barrel. What happens is that you bullets now have a different set of ballistics due tot he slower speed at the muzzle and require a greater hold over. Without knowing the exact speed I cant give you a drop table, but I could point out that the 30-06 is very similar to the x54r and should follow suit in velocity drops and term ballistics.
Nothing makes me feel quite like a man than beating on a midget. -Thed

Anxiously awaiting your next thread describing how you drown puppies in your basement. -Spinich

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #6 on: December 29, 2010, 01:51:03 AM »
What if we used some hypothetical numbers. Lets say the cartridge is 147gr LB Russian Surplus FMJ and leaves the muzzle of a standard M91/30 at 2850fps. Cutting the barrel to 20" drops the velocity 250fps and cutting it to 16" drops it 350fps. Does that give you enough to work with?

Danjal

  • Not the droid you're looking for.
  • Life Member
  • SKS Guru
  • *****
  • Posts: 15794
  • Who I am /\. Things I say >
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #7 on: December 29, 2010, 03:57:19 AM »
Still need the bc or an original drop table to work with. I can fudge the numbers from there to get it more exact. But I ran the numbers for ya for the M80 projectile. .391 BC for M80 (147 gr .308) Its the standard NATO ball for .30 cals. Id imagine the russian equivalent wasnt far off from that unless you're reloading the M80 projo itself.



Ballistics Calculation

Input VariablesFirearm type   Long   Sight Height   1.5
Bullet Weight (grains)   147   Ballistic Coefficient   .391
Muzzle Velocity (fps)   2500   Temperature   72
Barometric Pressure (hg)   29.53   Relative Humidity   78%
Zero Range (yards)   100   Wind Speed (mph)   5



Ballistics Table in Yards   147 gr., .391 B.C.   www.hornady.com

Range (yards)   Muzzle   100   200   300   400   500   600   700   800   900   1000
Velocity (fps)   2500   2289   2088   1898   1720   1554   1405   1275   1167   1084   1021
Energy (ft.-lb.)   2040   1710   1423   1176   965   789   645   530   445   384   340
Trajectory    -1.5   0.0   -5.2   -18.3   -41.1   -75.8   -124.9   -191.7   -280.2   -394.1   -536.8
Wind Drift (in)   0.0   0.5   2.0   4.7   8.7   14.3   21.6   30.8   41.9   54.9   69.5
Come Up  MOA   -1.5   0.0   2.5   5.8   9.8   14.5   19.9   26.2   33.4   41.8   51.3
Wind Drift (moa)   0.00   0.21   0.47   0.74   1.04   1.37   1.72   2.10   2.50   2.91   3.32


Ballistics Calculator v1.0
Hornady Mfg, Inc., by ARIS, Inc.
Nothing makes me feel quite like a man than beating on a midget. -Thed

Anxiously awaiting your next thread describing how you drown puppies in your basement. -Spinich

Aye Kay

  • SKS Guru
  • **
  • Posts: 3054
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #8 on: December 29, 2010, 08:35:57 AM »
  It's still gonna hurt.  I'll take a short rifle over a long one anyday. 

  I meant what you shoot.  I sure prefered to tote around my Enfield Jungle Carbine more than my standard length ones.  It hurt my shoulder more too now that I think of it.  Gave me a nice scope cut first time I shot it too.  :) 

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #9 on: December 29, 2010, 10:55:19 AM »
Yeah, I had a Jungle Carbine once and ended up selling it unfortunately. Kind of thinking along those lines with this MN I am playing with. The stock and barrel came cut down already and I was thinking about shortening the barrel a little more to make it look more proportionate.

Danjal, I remembered about 7.62x54r.net having ballistic charts on the known 7.62x54r surplus. It compares different MNs as well so it gives a good comparison between an M91/30 and an M44. I should be able to figure the drop off of that for going to a 16"-18" barrel.

       Case                                                                                 Over all Length    Powder
Headstamp                       Material    Coating        Water Volume     Min.      Max.     Weight
"60"(factory)/ "86"(year)    steel      copper wash      63.6 gr.            3.009"  3.017"   47.9 gr.

 
Bullet Dia.   Length   Weight       Style      Core     Tip Color  Magnetic? Ballistic Coefficient Sectional Density
.311"            1.264"    147.8 gr. spitzer bt    steel      none    yes           0.325                  0.219

Measured Muzzle Velocity in Feet per Second (10 shot string)
Rifle                            M44        M91/30        
Low                            2577        2779        
High                            2652        2897        
Average                      2629        2812        
Extreme Spread               75        118        
Standard Deviation           30        37        
Trajectory and Drop Graph
M44
   

M91/30


Bullet Velocity to 500 yards in Feet per Second
Distance (yds)    0       50       100      150       200    250      300      350      400     450     500
M44                2629    2494    2362    2235    2111    1992    1876    1765    1660    1560    1464
M91/30            2812    2671    2534    2402    2273    2149    2028    1910    1798    1692    1590

Bullet Energy to 500 yards in Foot Pounds
Distance (yds)    0       50       100      150      200     250      300     350     400     450     500
M44                 2271    2043    1833    1641    1465    1304    1156    1024    906     799    704
M91/30            2598    2344    2110    1895    1698    1517    1351    1199    1063    940    830
« Last Edit: December 29, 2010, 01:47:10 PM by Greatguns »

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #10 on: December 29, 2010, 02:04:24 PM »
I am speculating, based on the difference of an M91/30(28.75" bbl) and an M44(20.25" bbl) being 8.5" and cutting it to 16" being another 4.25"(1/2 again the distance), if I take the differences on the stats between the two rifles and multiply by 1.5 it should give me an approximate idea of what I would have with a 16" barreled MN. I would have approximately a 70" drop at 500yds, somewhere around 2500fps at the muzzle, around 1375-1400fps at 500yds, and bullet energy of around 2125 at the muzzle with 630 at 500yds. What do you think Danjal, does that sound close to you or is my math all whacked?

Aye Kay

  • SKS Guru
  • **
  • Posts: 3054
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #11 on: December 29, 2010, 02:08:43 PM »
   Hey Greatguns,  I have a magazine where a guy made a "Jungle Arisaka".  He had a gunsmith shorten and add a Jungle Carbine's flashider.  I wonder how a Nagant would look Junglefied?  :)

Greatguns

  • Board Supporter
  • SKS Guru
  • *****
  • Posts: 8937
Re: Ballistics question on 7.62x54R when shortening a barrel
« Reply #12 on: December 29, 2010, 02:23:55 PM »
It would probably look pretty cool, but I've always heard those FHs aren't very effective as a MB which is probably why they are known for their kick. I was looking at an AK-74 style MB to put on there. But, on the other side of things, it would give me a front sight again. Hmmmmm, I'll have to think about that.