Wanted to check my line of thought on something

So generally when loading plated the usual rule of thumb is to use lead data or low to mid FMJ data if specific load data for you bullet is not available.

My question is, if you use max or near max FMJ data for plated you should just get an increase in velocity but not pressure?

Is this correct?

If I am not mistaken, the only way to increase velocity is to increase pressure.  You could also reduce friction and get a bit more velocity with the same pressure, but I don't think that happens with plated bullets compared to FMJ.  The reason you use lower charges with plated is because they act more like lead, and form to the barrel more readily.  With the better seal, FMJ powder loads can cause an over pressure condition, or the higher velocities will just cause the plated bullets to fail.

Both jacketed and plated bullets vary in terms of the hardness of the lead core and the hardness and thickness of the jacket.

In many cases, bonded "jacketed" bullets are actually bullets that are plated to the same thickness as an extruded copper bullet jacket, so the major difference is in jacket thickness.

The core hardness can vary on both jacketed and plated bullets, and both core hardness and jacket thickness will have an impact on pressure.

The bearing surface of the bullet and its initial diameter also have an impact on how much force is needed to start the bullet in the rifling and in turn how high the peak presume may be.

Softer bullets also obturate faster in the bore at any given pressure, trapping more gas behind the bullet and potentially increasing pressure as well.

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The short story is that there isn't a simple way to predict pressure based solely on "plated" versus "jacketed" bullet construction.

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The rest of the story is that plated jackets tend to be much thinner and at higher pressures and velocities the plated jacket can be damaged or even partially stripped off the bullet.

For example, I shoot heavy plated 150 gr Xtreme bullets in my 1885 High Wall in .30-30 and I can get 10 shot 1.5 MOA groups with that combination.   However that assumes I keep the velocity under about 1775 fps.  As the velocity goes beyond that, I start to see an increasing number of significant fliers landing 4-6 inches outside the group at 100 yards.   The rest are still just as accurate, but the bullets that suffer damage at the higher pressure and velocity are wildly inaccurate.    

That's the primary reason most folks recommend using cast bullet data, or starting in the middle of the range when using load data developed for jacketed bullets.  

Xtreme recommends keeping velocity under 1500 fps with their plated bullets.

Barrys is like 1250 fps or less.

You might find this article of interest: http://38super.net/Pages/How%20Fast.html

It seems some plated bullet makers are getting better at telling you what the limits are of their bullets.

I loaded some Berry's 155gr FP .401 (40 S&W) and was surprised to see a max velocity of 1250 fps printed on the box. That is maximum jacketed velocity according to my Hornady manual.

As far as what difference there would be between a jacketed FMJ and a plated with the same load goes, there are too many variables to say.

Also a slicker, or easier to go down the bore, bullet very often is slower not faster. When moly coat was the latest fad many people found that the slicker moly coated bullets were shooting slower than the exact same bullet and load without the moly.

I recall at least one member here complaining because of the velocity loss in his AR-15 and there was not enough case capacity to regain it.

I personally experienced it with 200gr Nosler BT in .338 Win Mag. My home moly coated bullets were slower than the non-coated.

The reason is pretty simple. There is a lot of pressure rise while the bullet is being forced into the restriction of the barrel. A softer or slicker bullet requires less pressure to get it into the bore. After in the bore most of the resistance is gone, even more so if the bullet is slicker.

So the "harder to start" bullet builds more initial pressure and increases in bore time enough to generate more pressure which results in more velocity.

If you try to push a plated bullet beyond its design strength it will not be able to "hold" the rifling and will strip out like over torqued threads on a bolt.

Motor

9mm Xtreme plated

All weights

I should have been more specific

So let's say  I loaded a hornady 147 grain fmj to max load of a specific powder and it averaged 950 fps.

Then say I loaded an extreme plated to with the same exact charge and c.o.a.l and it averaged 1100 fps.

I would think that the pressure wouldn't be higher in the plated round than the fmj round

You are still losing me.  If the velocity is higher, the pressure must be higher.  How else do you propose the velocity would increase with less pressure pushing on the bullet?

It's even a little more complex than has already been discussed.  Berry's bullets have two different plating thicknesses.  RMR has a line that has "twice the thickness" of plating as most other plated makers' bullets.  Xtreme states they have "thicker plating" than "others."  What it comes down to is that different makers' bullets will handle different velocities differently, so consult the manufacturer's information before doing something "extreme" with plated bullets.

I don't think pressure is the issue you need to worry about with plated bullets.  It's whether or not the bullet will hold together at the velocity you push it to.  If it doesn't, you'll get buildup in your barrel and that can cause pressure problems.

I love this combo

There are loads with different powders that have a higher listed velocity but I've tried most and power pistol is still the king IMO