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barrel wear with subcalibers   Small Arms <20mm

Started 22-Dec by smg762; 2446 views.
smg762

From: smg762

25-Dec

The case size would be noticeably smaller than 556.  

556 has 1360lbs from a 20. 7barrel. 

Mine requires only 1100 from the same barrel. 

Even the tiny 204 vartarg will hit 1100. My round is slightly bigger than the vartarg but it would certainly weigh only about 7grams with polymer case. 

Lower barrier penetration is an accepted sacrifice... it's outweighed by the massive range and weight efficiency. 

Judging by the vartarg (400ft lbs at 350m),  I estimated 450ft lbs at 400m. 

This is probably way off,  but I was using the crap ballistics calculators on www. Shooterscalculater. Com

gatnerd

From: gatnerd

25-Dec

smg762 said:

This is probably way off,  but I was using the crap ballistics calculators on www. Shooterscalculater. Com

This is a good free one:

https://bergerbullets.com/ballistics-calculator/

smg762

From: smg762

25-Dec

I tried it.  Entered my new heavier bullet at 52grain.  

I had no idea what the final B.C. would be.  It's a very long bullet but has no ultra long ogive...  boat tail is aggressive but I can't use a super-good ogive because most of the bullet is buried in the case....    The cartridge OAL must be short (46mm) because it's also chambered in my pistol-PDW hybrid. 

ANYWAY I decided to be cautious and enter an average B.C. of 0.29 (that's the number I put in the box)

I didn't know whether to use the G1 button or the G7 one. 

Trying the G1,  i got 310ft lbs at 500yard. 

230,  at 600. 

Trying the same with the G7 button,  I got some crazy performance.... about 550ft lbs at 600 yards.... and it could reach out to 1000yards. 

This seems unrealistic,  even though 0.190'  inch bullets have superb range. 

So I don't know why there's a big difference between G1 and G2...  and I don't know if I even used the program properly

  • Edited 25 December 2020 9:11  by  smg762
In reply toRe: msg 23
smg762

From: smg762

25-Dec

I get the same G1-G7 disparity,  when I use their 224VLDs,  or 204VLDs

The g7 gives far higher performance. 

Also they list their super efficient bullets (like the 80grain 224) as having BCs of roughly 0. 21

I always thought a B.C. of 18 to 21 was poor...  that's why I entered 0. 29

  • Edited 25 December 2020 9:40  by  smg762
EmericD

From: EmericD

25-Dec

A 52 gr bullet of .19" diameter will have a 0.206 lbs/in² sectional density.

With a short ogive and a long boat-tail, the i7 form factor will not ne lower than 1.15, so the best G7 BC you can expect is around 0.18, or around 0.36 if you use the G1 reference.

smg762

From: smg762

25-Dec

Tried the G7 option with. 18bc,  it gave a rather optimistic 340lbs at 600 yard. 

Bear in mind the muzzle is 1100lbs. 

The G1 gave me an error message.  Said I had entered the wrong inputs. 

JPeelen

From: JPeelen

26-Dec

Keep in mind that the G1 ballistics was derived from rather blunt naval artillery projectiles of the 19th century. G7 ballistics was derived from projectiles with modern, pointed ogive  and boattail (British firing trials of 40 mm and 76.2 mm projectiles in the 1940s). 

The persistent popularity of G1 within the shooting community is based on the effect that a G1 BC (when measured over the first 100 yd or so) results in much "better" velocity figures at longer ranges than G7. Example for 55 gr Lapua S538 at 300 m: G1 605 m/s, G7: 580 m/s. One tends to accept 605 m/s, but the radar measured range reality is 580 m/s (muzzle velocity 955 m/s).   

Therefore, G1 gives misleading results for bullets of modern, slender and pointed shape. The other important thing is to use the correct BC. For a given bullet, it is not the same for G1 and G7, as the example by EmericD shows. Always make sure you use G7.    

smg762

From: smg762

26-Dec

Thanks yeah all I'm trying to do is get an accurate gauge of the 400-500m energies. 

I thought those G7 results were rather high, but all I could do was enter a. 18 in the B.C. box. The projo has a similar shape and length to the 4.7hk

do you think the 340ft lbs @ 600 yards was at all accurate... 

JPeelen

From: JPeelen

27-Dec

If a ballistics program is not clear about the drag model it uses, as seems to be the case here, I recommend to not use it. You never know how competent the programmer really is. 

A web program which was, I believe, written by competent people can be found on https://appliedballisticsllc.com/ballistics/index.php

EmericD

From: EmericD

28-Dec

I'm not very familiar with ft-lbs, but it seems that you want a cartridge delivering 1490 J at the muzzle, when firing a 52 gr bullet (3.37 g) of 0.194" diameter (.19 calibre).

That's an equivalent of a 80 gr / .224" bullet, and according to Bryan Litz database, the i7 form factor of those bullets are between 0.98 and 0.99 for the Berger VLD, the Hornady A-max and the JLK, so you could expect that the G7 BC of your 52 gr bullet will be 52/.194^2/7000/0.99 = 0.199 if you allows for a 2.6*0.194 = 0.5" ogive height (the cartridge OAL should be at least 0.5" longer than the case length).

If the difference between the COAL and the case length is only 0.42", then the expected i7 of the bullet will probably rise from ~1 to 1.15, and the G7 BC will drop from ~0.20 to ~0.17.

If you aim for a MV of 1490 J, then the MV should be ~940 m/s, so here are the results for the 2 form factor hypothesis:

G7 BC = 0.20

0 m - 940 m/s - 1488 J - 1097 ft-lbs

300 m - 709 m/s - 847 J - 625 ft-lbs

600 m - 513 m/s - 444 J - 327 ft-lbs

G7 BC = 0.17

0 m - 940 m/s - 1488 J - 1097 ft-lbs

300 m - 672 m/s - 761 J - 561 ft-lbs

600 m - 451 m/s - 343 J - 253 ft-lbs

So, depending on the space available for the ogive, you could expect between 250 and 330 ft-lbs of KE at 600 m.

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