The Mk VIIIZ round

[CINDERS]

Found this article rather interesting seems the MkVIII's projectile had a BC of .467 as apposed to the SMK at .499 when driven at 2,200FPS +.
Interesting to note the card disc apart from stopping anti-ablation also stopped to a degree the torch affect on the throat from the cartridge being fired.
Its a bit long but well worth the read I found it interesting;

.303 British, Mk VII ball & the “bore-killing, hot Mk8Z”

So, here I was somewhat bored, surfing the various Enfield forums & I got to wonder about some ammo I had acquired. It was headstamped as Prvi Partizan (“nny-85” headstamped) Mk8Z in .303 British. This is the “bore killing uber hot machine gun ammo, never to be used in rifles”. What was this stuff in reality?

Being me, I pulled down a couple of dinged up rounds & did some testing & guess what I found? There’s more than one kind of “Mk8Z” & it differs quite a bit by country/manufacturer. A bit more research & I worked out that a lot of us are actually making & shooting modernized Mk8Z-type ammo & our bores are doing just fine.

OK, let’s start at the beginning. What’s “real Mk VII” in detail?

In 1910 the British "War Office", part of the "Ministry of Defense", “sealed” the design & nomenclature of British Mk VII service ball ammunition. “Sealed,” means no changes, ever the spec is cast in stone & can’t be un-done, or modified without some serious pre-planning. It’ll probably become a new Model, Mark, or at the least get a (*) added to denote it’s different from “regular issue stuff”.

Such was the case with the MkVII (or Mk 7, after 1945) ball ammo. It was a 174 Gr FMJ flat-based bullet with a “tenite, (cellulose plastic)” fiber or aluminum internal core at the front & a lead core behind that. B.C. was 0.467. Propellant was charge weight that could vary from 35 to 38 grains, averaging 36.5 grains of “Cordite”, (the standard 58% Nitro-glycerin, 37% Nitro-cellulose and 5% Mineral Jelly) propellant of most British ammunition. Muzzle velocity was 2440 FPS & pressure was 52,900 PSI. (Current SAAMI specs are lower at 49,000 PSI, but that was yet to come, as were modern measuring techniques for pressure). 45000CUP was the actual measurement used at the time. There was a card disc over the propellant used as an anti-ablation shield to reduce flame-cutting erosion of the bore.

So. What’s different about a MkVII(7) or Mk VIII(8), or Mk8(z) round, designed for use in the Vickers machine gun for long range shooting?

A few things actually & there are several variants on the basic Mk8 Mk8(Z) design. It was never “sealed” like the Mk VII(7) was, so changes were easy.

The biggest difference was the bullet. The ballisticaly inefficient Mk VII round with its drag inducing flat base was replaced by a couple of different boat tail designs. Boat tails reduce drag, increase the BC of the bullet & allow it to fly on a flatter trajectory, increasing the effective range over a similar flat base one. The Mk 8 bullet was a 175~190-grain boat tailed, streamlined, steel jacketed bullet. Initially the boat tail was not full diameter but “stepped” with a flat annular base with the boat tail starting at a slightly reduced diameter. This was done in an attempt to stop rifling erosion, resulting in blow by & reduced accuracy. Because of its design the Mk 8 bullet was found to have a different issue, stiffness. Compared to the relatively soft Mk 7 it didn’t obdurate as well, so conforming to the rifling, particularly in worn, or cordite-eroded 2-groove barrels, was less efficient. At that point the stepped boat tail was dropped in favor of a “normal” full diameter boat tail design.

Slightly more “neonite”** propellant was added, giving a muzzle velocity of 2550 ft/s (780 m/s) and somewhat better ballistics. Chamber pressure was higher, at 40,000–42,000 lbf/in² (about 280 MPa). These were the “Hotter, Vickers machine gun only” ammunition that started the whole debate off. Neonite** incidentally is NOT cordite, nor a derivative of it. The closest current propellant to it is IMR 3031, a nitro-cellulose powder!

But there wasn’t anywhere near the standardization that existed with Mk VII ammo because of the “sealed changes” in design!

So is ALL Mk8Z a bore killer, or are there variations that are less damaging?

Some surprising things I’ve discovered about Mk8Z ammo. It varied way, way more than thought, look at these pull down figures!

Mark VIIIz Spec - bullet 175~190 grns +-2 grns, propellant 37 to 40 grains “neonite”** . Velocity 2,360~2,550 fps.
Spennymoor 1941 - Bullet 175.0 grns, prop.38.0 grns
RG 1944 - bullet 176 grns, prop. 36.5 grns
Spennymoor 1944 - bullet 174.5 grns, prop. 37.5 grns
Spennymoor Ballistic Standard 1944 - bullet 174 grns, prop 36.5 grns
RG 1945 - bullet 174.5 grns, prop.36.75 grns
RG 1948 - bullet 177 grns, prop 37.0 grns
IVI 1985 - bullet 175 grns, prop 37.5 grns
(The ballistics of the IVI Mark 8z was arranged to be between the Mark 7 and Mark 8z suitable for the No.4 rifle with 300 yard battle sight).

I weighed & measured the COAL of a S/A "R1M3Z ~ A80" (which is Mk VII spec, including the filler-tipped 174 Gr bullet), & the "nny-85" ball round, heres the results for loaded, unfired rounds:
R1M3Z
Length = 3.0285"
Weight = 408.7 Gr.
Powder charge was 39.6 Gr of a stick powder similar in appearance to IMR 4895, but there’s no way to tell what exactly it was. The bullet was a FMJ FB 174 grain MKVII.

nhy-85 Mk8Z
Length = 3.053"
Weight = 405.5 Gr
Powder charge was 38.4 Gr of a stick powder similar in appearance to IMR 4895, but there’s no way to tell what exactly it was either. The bullet was a boat tailed 190 gr FMJ.

Now for the big surprise! The Prvi Partizan MkVIII(z) variant is slower than South African R1M3Z. It also has less recoil (big surprise there.)
Chronograph results.
R1M3Z 174Gr ball
Average = 2525
Low = 2497
High = 2592

Prvi Mk8Z 190 Gr ball BT
Average = 2359
Low = 2335
High = 2359
(All velocities instrumental @ 10' from a standard No4 Mk2 barrel).

The SA R1M3Z is about on the ball for weight. Military .303 weights are 174gr bullet, 195 gr case and about 35-40 gr prop. With 38gr prop and a 190 gr bullet it suggests the PPU has a case of about 177 gr.

For comparison purposes, case weights & thickness vary a lot so its misleading to just weigh a loaded round to see whats insde it:
empty (fired case) weights
S/A R1M3Z = 201.4 Gr
nny = 178.2 Gr
R-P commercial = 160.9gr
PPU = 173.3gr
HXP = 186.7
RG-50 = 185.7 Gr.
(This is actual British-made Mk VII ball, so I include it as a reference for weights.)

So all Mk8Z is not created equal at all!

Bullets can vary in weight, velocities likewise & several different propellants are used. The reason cited for the “Machine gun only” restriction was because of the combined effects of cordite erosion (amazing as there was no cordite in them) & boat tail bullet shape & construction. Not one cause, but a combination of things. Flame temperature, bullet construction & bullet shape all combining to have a negative & damaging effect. Where does that leave us today? There have been improvements in powders, better designs of bullets & so on. Can we make, or use something with the better performance of the “Long Range Mk VIII(z)” or are we stuck with the Mk VII including it’s flat base limitations?

Well whats a”regular, average, normal” Mk8(z)?
Using the widely varying components & velocities of the different versions of the round, somewhere between a 175gr & a 190 gr FMJ BT with a BC of between 0.470 0.485 & bullet doing somewhere between 2550FPS at the muzzle & 2359 FPS at 10 feet with somewhere below 42000PSI of chamber pressure.

Heres my suggestion. We not only can do a better MKVII(z) but we have been doing it for quite a while with a great deal of success & no horrible barrel damage.

How ?

Use the Hornady .311” 174 gr Matchking, or the similar Hornady #3131 .3150" 174 Gr FMJ-BT!

Propel it with 41.6 gr of IMR 4895 for 2450 FPS, or 43.0 gr of .IMR 4320, for 2470. None of these exceed the lower SAAMI pressures of 49,000 PSI. & The Sierra Match King’s BC of 0.499 & better construction will allow full upset for full obduration of the bore completely eliminating the blow by problems of the old Mk VIII bullets, while gaining the trajectory because of its better design & boat tailed shape. The lower flame temps of modern powders should help as well. In fact they have been doing just that at matches for years, we simply didn’t recognize it!

**”Neonite” is a nitrocellulose propellant, as referenced in both 1920's "Dictionary of Explosives" as well as the 1944 edition of the "Textbook of Ammunition".
Neonite loaded ammunition, both pistol and machine gun, is listed with the 'z' designation, the chopped cordite loads for pistol ammunition being clearly differentiated from it.

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[jonh172]

Wow!!!

Thanks for sharing!

Having just started down the road to 303 long range shooting/reloading, this answers so many questions and gives the confidence to reload the way i was hoping too and know I won't be hurting my T's.

Invaluable to new 303 shooters!