I've got some Woodleigh 180-grain PPs to load in my 270 WSM. However, since the bullet's body seems to extend well forward and the Tikka's throat is not overlong, the bullet has to be seated about half an inch in beyond the neck.

Does this lift pressure as the gas works on the side of the projectile while bearing on the base?

No. Only the effective powder capacity is germane. Not the fact that the bullet is touched by powder. Think about it; the bullet can't move sideways as it has equal pressure all around it, but it can only move forward.
Tank ammunition has the projectile sitting on the primer and surrounded by propellant. Sabot rounds have to be long to work.

Having the bullet compress the powder really depends on the powder burn rate and does the bullet damage the powder grains deterrent coating or turn the powder grains into fine particles.
In the latter, this would greatly increase the powders intended burn rate causing the chamber pressure to spike.
That being said, today's powders are almost always made to a military specification and the powder grains are pretty tough.
Few powder charges are filled to the top of the case neck with most at or slightly below the case shoulder. But having the bullet go into the case 1/2" the powder compression is really not to the extreme.
If pressures exceed the norm, you will see a flattened primer or a stiff bolt on opening.
You don't mention what powder you will be using?

No one mentioned compressing powders, nor exceeding proven load data. The question as I read it, was simply, does having the bullet protrude into the powder chamber, affect pressure. (OP specifically asked about pressure on the side of the bullet)
Given a proven load the answer is, no.
Of course if you take a load tested for a bullet seated way out, and then jam half of it into the powder, that changes everything in a bad way, but not because there is pressure on the side of the bullet.

quote:

Originally posted by dpcd:
No. Only the effective powder capacity is germane. Not the fact that the bullet is touched by powder. Think about it; the bullet can't move sideways as it has equal pressure all around it, but it can only move forward.
Tank ammunition has the projectile sitting on the primer and surrounded by propellant. Sabot rounds have to be long to work.

I agree. Your effective pressure vessel is the capacity of the chamber when the bullet engages the lands. The longer the bullet the smaller your pressure vessel. Clearly, having to seat a bullet deeply has the effect of reducing the volume of the pressure vessel (many people errantly refer to this as "powder space", but we all know what we're talking about.) And just as clearly, a reduced pressure vessel requires an appropriate reduction in propellant to keep pressures at the desired level.

The necessity of deep bullet seating with the Short Magnum rounds is one thing which limits their versatility compared to more conventional rounds.

True which is why I have no need for short actions or cartridges and see zero advantage to them, in spite of their popularity.
I say if you can't operate that bolt the extra 3/8ths of an inch, you probably shouldn't be shooting.
Sorry, off topic.

quote:

Originally posted by dpcd:
True which is why I have no need for short actions or cartridges and see zero advantage to them, in spite of their popularity.
I say if you can't operate that bolt the extra 3/8ths of an inch, you probably shouldn't be shooting.
Sorry, off topic.

Maybe off-topic, but I hadn't thought of that and I appreciate the insight. So I'm off topic too.

I have loaded H 870 in the 300 win mag to 83? grains I'd have to check but it was full to the top and then I tapped the case to make it settle and topped it off again resulting in that charge weight. I'd have to double check that number but but H870 has very low pressure.
And that was the only way to get respectable velocities with it.

True; H870 burns like black powder, but I think the OP's relevant number is 270, rather than 870.

Thanks everyone,
So, I glean that if there is a problem it will be because I have effectively reduced the case capacity, and that that could lift pressure.

The powder is AR2213sc, which I believe is H4831 by the maker's own name. The Woodleigh loads started at 53 grains (yielding 2560fps) with max. 56gr (2670). However, concerned about the intrusion, I started at 52.5gr and only loaded up to 55.5gr.

I don't believe there was any shortage of space and, considéring that up to 73 grains is listed for bullets half that weight, hope that the protrusion doesn't exceed 20 grains of powder room.

How did I come to use such a long, heavy bullet in this short, ugly case? Well, hearing the Woodleigh factory had burnt down, I started combing the shops for remaining stocks. It was slim pickings for all the big bores but plenty of .270 bullets remained.

Fifty years ago when I first ordered a .270, I'd wondered about the aftermarket 160 and 170-grain projectiles - but the Husqvarna 4000 never arrived and my interest in .270s wained until planning to go tahr hunting in 2010.

So, considering fallow deer are now common in our sambar country, you never know what will appear. But if when hunting fallow early in the year I come across some red-hot big sign, I'm thinking it might be good to load for sambar - which grow larger and tougher than black bear, at least

Seating depth of heavy for calibre bullets can be limited by the ogive shape too where you will loose neck tension when seating certain bullets deep in the case to ensure cartridges fit the magazine. As others have said, if case capacity is reduced by seating bullets deeper then pressures will be raised. There is no rule for seating depth other than the limits of neck tension and remaining case capacity. Small case capacity and heavy bullets just achieves lower intensity cartridges e.g. 35 Remington etc That is not what the 270 Win, or WSM, is about.

I personnally can't see any need for bullets heavier than 150gr in any 270 let alone the WSM. The lighter well constructed bullet will kill quicker than the slower heavier bullet on any game.
I will be loading for my recently acquired 7mmWSM and won't be loading anything heavier than 160gr, in fact will likely go with a good boattail 150gr to get a better combination of velocity and flat trajectory particularly for hunting Tahr.
Jack O'Connor wrote that when using a 7mm Rem Mag on African game he got quicker kills with 150gr bullets than with 175gr bullets.