Your definition of “back yard friendly”

If the cops aren’t there in 15 minutes... 

Seriously though, the gun will always be louder to the shooter.

If you have the gun and you are concerned what the neighbors might hear, the gun unloaded of course for safety reasons, have someone discharge the gun while you are standing near where you believe they would hear it. Remembering they would just about have to be outside or have windows open to hear most PCP guns anyway.

Would the report be concerning to you?

Could someone discern what the noise is? 

If you don’t have the gun I’d suggest you arrange to reduce the repot as much as possible. And know that the larger the caliber the louder the report.
But, this all only refers only to the noise the gun makes.

The absolute last thing you want is a stray pellet beyond your property. Take all necessary steps to insure this does not happen. My trap is rubber mulch 6” deep.

I live on an acre with no fence. Our ‘hood HOA sez no privacy style fences except pool enclosure. Only split rail type. 

The biggest concern I personally have is, being seen with the gun. I have a incognito pellet trap at the end of my property with access to the green belt and shoot from inside my garage. 

Remember, safety first.
 
"davecole"If the cops aren’t there in 15 minutes... 

Seriously though, the gun will always be louder to the shooter.

If you have the gun and you are concerned what the neighbors might hear, the gun unloaded of course for safety reasons, have someone discharge the gun while you are standing near where you believe they would hear it. Remembering they would just about have to be outside or have windows open to hear most PCP guns anyway.

Would the report be concerning to you?

Could someone discern what the noise is? 

If you don’t have the gun I’d suggest you arrange to reduce the repot as much as possible. And know that the larger the caliber the louder the report.
But, this all only refers only to the noise the gun makes.

The absolute last thing you want is a stray pellet beyond your property. Take all necessary steps to insure this does not happen. My trap is rubber mulch 6” deep.

I live on an acre with no fence. Our ‘hood HOA sez no privacy style fences except pool enclosure. Only split rail type. 

The biggest concern I personally have is, being seen with the gun. I have a incognito pellet trap at the end of my property with access to the green belt and shoot from inside my garage. 

Remember, safety first.
I agree. Also if you have the wrong neighbor that freaks out about stuff, nothing is backyard friendly. Anyone in their backyard in a "normal" sized neighborhood could figure out what you are up to even shooting the quietest gun. So I would say backyard friendly really means.... How friendly are you with the neighbors?
As for the Royale 400...... It is very quiet for a pcp in my opinion. I would say about the same as a Red Ryder BB gun. Just a little sharper and tighter. Definitely quieter than any springer I've heard.
 
Thanks for the comments. My two neighbors are about 100 yards from where I would be shooting. The back neighbor has woods between me and them. The side neighbor gave me an OK to shoot. Just don’t want to disturb there privacy if you know what I mean.
Rhode Island designates a Airgun as a gun and silencers are Not legal in that state. RI and New Jersey are the only two states to do this. I hate this state !!!
 
The way I got neighbors to be happy with my backyard shooting was I lied and told them that there were rats in the woods and I killed one that was eating out of my dogs dish. So they now think I’m great for getting rid of the neighborhood rats and no complaints about noise. But I’m not shooting early in the am which would get some of the late risers mad.
 
"Grandfather"Thanks for the comments. My two neighbors are about 100 yards from where I would be shooting. The back neighbor has woods between me and them. The side neighbor gave me an OK to shoot. Just don’t want to disturb there privacy if you know what I mean.
Rhode Island designates a Airgun as a gun and silencers are Not legal in that state. RI and New Jersey are the only two states to do this. I hate this state !!!
No moderator............. It will be loud!
 
Backyard friendly:

1. Quiet gun that cannot be heard from inside their house - pretty much every air gun.

2. Shoot from inside your house, through a window with screen removed.

3. Always shoot from a location higher than your target so bullets will stop in the target or in your yard. 

Most air guns dont sound like guns, but more like an air tool or nail gun. Nobody has questioned me, and I shoot from my second story bathtub window.
 
I think of power first. Any pellet of any power level can be dangerous if it leaves the property, but some are more dangerous than others. I don't consider any .25 truly backyard friendly, although I most certainly have shot my .25 Marauder in an urban backyard, albeit always with a heavy backdrop and always back towards my own house so that if something goes wrong, it will hit my house and not my neighbors'. 

I consider my .177 Marauder at 12fpe very back yard friendly. Even so I still consider my backdrop when I shoot it in urban environments.

Now that I live in the woods I don't have to consider my neighbors near as much (nearest neighbor is half a mile away as the crow flies). 
 
Anything that doesn't distract or irritate your neighbour is my definition of "back yard friendly". The answer to this is to ask your neighbours.
-Someone claimed at this forum that his gun was far more back yard friendly than his neighbours wife. Never the less, he had to give up his shooting, the neighbours wife never gave up her nagging......
 
If in doubt, put a video camera at 40yds or the minimum you intend to shoot and shoot over your camera. Make the pellet hits a safe back stop another 25yds or further. And listen to how your gun sounds before the pellet hit the target. Best letting the pellet hit further out if you can safely achieve that.
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Or watch my video where I shot over my camera at 35yds and another at 88yds with same gun but with a Huggett. You'll be surprised how quiet our airguns really are. Have fun!

http://www.airgunnation.com/topic/a-pellets-whisper/