Can someone explain please HPBT & FMJBT?

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All handloading data posted on Full-Bore UK from 23/2/2021 must reference the published pressure tested data it was sourced from, posts without such verification will be removed.
Any existing data without such a reference should be treated as suspect and not used.

Use reloading information posted here at your own risk. This forum (http://www.full-bore.co.uk) is not responsible for any property damage or personal injury as a consequence of using reloading data posted here, the information is individual members findings and observations only. Always verify the load data and be absolutely sure your firearm can handle the load, especially older ones. If in doubt start low and work your way up.
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Victory Glow
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Can someone explain please HPBT & FMJBT?

#1 Post by Victory Glow »

Hello folks,
I know the physical differences between a HPBT and a FMJBT, but if you have the same weight bullet would one be able to use the same load data? I’m guessing maybe / maybe not due to the slight difference in shape at the point of the bullet which would affect the ballistic coefficient (i.e. the speed and the way the bullet travels through the air?) Could some kind person with more knowledge than me explain? Thanks, VG
1951 Winchester Model 70 Heavy Barrel 30-06, 1943 Mk4 1* Savage 303, Marlin 1894 357, 1943 K98 7.92, Anschutz 1416, Browning SA22, .30 M1 Carbine straight pull, Benelli & Browning B25 Shotgun, Walther LGV Pro .177
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WelshShooter
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Re: Can someone explain please HPBT & FMJBT?

#2 Post by WelshShooter »

Not always!

If you are comparing two identical projectiles, the only difference being the tip profiles, the load data should be the same. The main differences you'll find in load data is between bullets by different manufacturers. Load data for a 155gr 30cal bullet are different for a Lapua Scenar versus a Sierra Matchking. This is due to two main parts: the friction of the outer jacket (possibly purity?) and bearing surface area which affect the pressure. See the diagram below.

Whilst the bullet weight and tip type can be similar if not identical to other brands, other dimensions such as ogive length and bearing surface area can have an impact. This is why you may see different load data and velocities associated with certain types of bullets. You'll have to decide where your desired bullet sits, but I would always work up from the lowest max charge as a "worst case" scenario. You can use a chronograph and see which bullet type yours follows best.

In the example of 308win with N140, the max load for a 155gr SMK and Scenar are 44.2gr and 43.4gr respectively according to vihtavuori's website, with a velocity difference of 64fps in Sierra's favour. Is that different is velocity due to difference in powder charge or due to geometry of the bullet? Either way the resultant pressure may be exactly the same despite this outcome.
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Re: Can someone explain please HPBT & FMJBT?

#3 Post by Laurie »

The type doesn't affect pressures and hence powder charge weights per se. However, two very similar looking bullets of the same weight may produce different pressures in the same barrel depending on their dimensions and construction. The main metrics are:

Shank diameter. For instance, military spec 7.62mm FMJBTs are often undersize compared to the industry nominal value of 0.3080" (7.82mm). NATO specced its FMJBTs at 0.3073 - 0.3077" IIRC (one reason why such bullets often group poorly in true 30-cal barrels (0.300" bore dia' ' 0.3080" groove dia.). Conversely, Sierra MatchKing HPBTs like many match bullets are manufactured slightly above SAAMI nominal size at 0.3083 - 0.3085".

Shank length (aka Bearing Surface Length or BSL). This varies between designs and all other things being equal, the longer it is the higher the pressure / the lighter the maximum charge weight.

Jacket thickness / brass alloy. The thicker the jacket, the higher the pressure. Likewise the 'Gilding metal' - a low zinc content form of brass - alloy content may make a bullet 'harder' or 'softer', that is creates more or less pressure when engraved by the rifling. The same applies to the lead alloy core which is usually a low % antimony alloy with the lead. The higher the antimony content, the harder the core and as with the jacket creates different pressures when squeezed down to fit the rifling.

Nose shape. Two bullets make look similar to the naked eye, but the ogive's position vis a vis the case-head/bolt may be different at the standard industry COAL thereby giving a different amount of jump to the start of the rifling.

As a general rule, these factors have much less effect than bullet weight usually making relatively small differences. There are exceptions though which can produce considerable differences in pressures and so vary maximum charge weights by more than the usual <1gn amounts. Undersize 7.62mm NATO bullets is one (why several generations of 'TR' rifles had undersize bore and groove dimensions as they were designed to cope with standard NATO issue ball ammunition). The occasional thick jacket / hard core type can produce a nasty surprise too. I suffered this many years ago when I was given a job lot of old and unshootable pre-WW2 .30-06 'M1' type ball ammunition that was loaded with a long-range 173gn FMJBT bullet. Using 180gn HPBT match bullet loads data to reload recovered / pulled M1 bullets with modern brass and powders, I soon starting blowing primers at what should have been only just above starting loads and had to start again demilling my handloads. Subsequent email correspondence with German Salazar, the US 30-06 aficionado / ultimate handloading expert confirmed that this bullet was made exceptionally 'hard' for some reason now lost in the mists of time (nearly the centenary of its adoption by the USA now) and that loads had to be reduced by up to 3gn.
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Re: Can someone explain please HPBT & FMJBT?

#4 Post by snayperskaya »

From experience, in the case of Russian 7.62x54r bullets the standard light ball (LPS) bullet weighs around 150gr.The T-46 tracer bullet is the same weight as the LPS bullet but is significantly longer and the subsequent difference in length means the tracer round charge weight is lower due to the increased length of the projectile and the increased pressure due to a larger bearing surface.
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Re: Can someone explain please HPBT & FMJBT?

#5 Post by WelshShooter »

Laurie here with another quality post. Pic related.
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Re: Can someone explain please HPBT & FMJBT?

#6 Post by snayperskaya »

signfunnypost
"The only real power comes out of a long rifle." - Joseph Stalin

Give a man a gun and he can rob a bank.....give a man a bank and he can rob the world!.

More than a vested interest in 7.62x54r!
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Re: Can someone explain please HPBT & FMJBT?

#7 Post by Ovenpaa »

Laurie, what happened to German Salazar, I used to read his Journal with interest and then one day it was locked and that was the end of it.
/d

Du lytter aldrig til de ord jeg siger. Du ser mig kun for det tøj jeg har paa ...

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Re: Can someone explain please HPBT & FMJBT?

#8 Post by Laurie »

AFAIK German is still alive and well in the absence of any conflicting information David. He suffered two problems some years back. First was what he called a four-letter word. W-O-R-K !! He's an Arizona lawyer and was finding it difficult to make time for his shooting activities and the blog. The other was he had shoulder injury that needed fairly major surgery making his first love - sling shooting - impossible during the recuperation period. He had a couple of Borden action Eliseo tubeguns built to shoot F/TR (in his beloved 30-06 which many US clubs allow in this discipline), but I felt the heart had gone out of his interest in competitive shooting. Then he just dropped out, the blog wasn't updated and after a while closed down. Some of it is still available in archive form - I have a link somewhere. Maybe his shoulder didn't heal well, maybe the day job became too onerous, maybe his friends and co-conspirators in the gunsmithing and feature research side dropped out, died, whatever. Who knows? I've never seen an explanation.

The one certainty is that it's a shame as he did excellent research and writing and his blog was a great resource to shooters and handloaders.
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