Hi!
can we make the difference between action and bar-back action without dismounted the lock plate.

not really....sometimes they have fake pins to confuse folks

With all due respect to gunut, I have never come across a fake pin on an English gun but no doubt they exist.
The rule of thumb is that if there is a pin showing in the bar of the lock plate, it is a true bar-action lock. If not, it is a back action masquerading as a bar-action, sometimes called a 'back-action bar lock'.
At the risk of stirring up a hornet's nest, it is usually assumed that a back action is of lower quality than a bar action.
These people are ignoring the many sidelock O/U's and double rifles made by the best gunmakers in the world that very often use back action locks.
Although I will admit that a larger proportion of full bar-actions are of high quality than back-actions, it does not follow that a back-action can not be of fabulous quality and finish.
As an example I would offer up this McCririck:
http://www.heritageguns.co.uk/McCririck%20SLE/McCririck%20SLE%20Details.htm
and any number of guns by Blanch and Scott.

"Back work" and "back work and bar" locks need less metal to be machined out of the action , one of the reasons they were used in heavy calibre rifles . Side lock O/Us used them as there was no space for conventional bar lock , guns like the Holland Dominion used them so to get a slimmer rounder body . Many Continental makers used them because that's the way they made guns .
Never seem a back work and bar lock with a fake pin so as far as I am concerned if it hasn't got a pin on the bar its a back work.

.....and a back action leaves more metal in the action for strength. The trade off though is a little more wood is removed inletting the lock. I think the bar action spring can be somewhat smoother and perhaps stronger than the other. But for hammerless guns it's probably a moot point.

This same post was on SGW today, by a different poster, and made no more sense over there...

I have seen some fake pins...don't think any were Brit...although I wouldn't say they don't exist...would think they would mostly be found on cheaper knock offs of higher quality guns....

lets not forget blind pin guns where none of the pins show on the outside....hard to tell them from a side plated boxlock.....

The story I was told was bar action locks in theory will have a better trigger pull due to the way the springs pressure is applied to the sear making it the better system....

"the bar" it means the front?
thank you

Granger:
The 'bar' of an action is the section that lies under the barrels with holes for the barrel lumps and the hinge pin at the front.
The 'bar' of a lock plate is the part of the lock plate that lies along the side of, and is inlet into, the 'bar' of the action.
The pin I mention is the locating pin on the main spring which, in a bar-action lock, is usually visible towards the front end of the 'bar' of the lock plate.
In a back-action, this pin is usually towards the upper tail of the lock plate but since it is usually indistinguishable from other pins such as the interceptor sear spring pin, this information is not terribly useful!

Agree with Toby. Spotting a bar lock just requires you yo look for the mainspring locating pin at the forward extremity of the lock.

A lot of cheaper guns were made as back-actions but so were lot of very expensive ones - the obvious being Purdey's island lock guns and Grant & Hodges patent side-levers, 90%+ of which seem to be back locks.

I find the weight in a bar lock is concentrated around the action a little more. All my personal hammer guns are bar-locks, not a conscious decision on my part but perhaps an interesting observation, they just feel right in the hand. That is just my opinion - the Marquis of Ripon pretty much exclusively used back action Purdey's - so what do I know!

I apologize for being pushy, do you have pictures To make it clearer to me. again I am sorry

and a question to small bore . What is " Purdey island lock "?

Just my opinion, but my 1907 back lock Holland is a joy to shoot with, easily as nice a balance and swing as my EJ Churchill XXV bar lock. I also note that H&H is still making best back locks, in guns as well as rifles.
Granger, afaik, island locks (and peninsula locks) are found in older hammer guns, where the lock plate is separate from (or only butting to minimally) the action body.

Granger:
Here is a Purdey with Island Locks. Note the lock is separated from the action by wood.


Here is a full bar action lock with the main spring pin clearly visible in the bar of the lock plate.



Here is a back action bar-lock where there is no visible main spring pin in the bar of the lock plate.
The gun was a Pigeon style hammergun and back action locks were used for maximum strength in the action bar.

The shape of the lock plate is irrelevant, look for the main spring pin.

Not all bar action hammer guns had the pin (screw post) in the mainspring area.
Picture of Quality AA L.C. Smith, one of two known to exist.

Instead of a pin, the area for the mainspring was milled out to half the height of the mainspring.

I refer to this type of lock, a 5 pin 2 screw bar action lock. This 5 pin was used from 1884-1889 when it was then changed to a 6 pin 2 screw bar action lock.
Only one true pin and that is the sear axle.

It is noted that both the L C Smith & Baker sidelock hammerless doubles have back action locks with a plate having the bar. Likewise does a British J P Clabrough & a German J P Sauer & Son of which I have an example of each. None of these would qualify as "Best" but they likewise are not "Cheap Junk", all being solid well built guns.

After seeing several muzzle loading hammer guns with busted lock springs that when they broke they had caused severe damage to the stock it dawned on me that one of the reasons a bar action is better is because should the lock spring break in a true bar action it's inclosed in steel and there will be no cracked stock...other than that I see no real advantage or disadvantage over other designs.

thank you all