I understand that there is wide range of bullet calibers. I am interested in the ones that is 50 caliber or less, and if you can be high enough in a blimp to be safe either by to far to see or to far to reach?
There is a XKCD What if article about catching bullets fired upwards (because why TF not, right?), and it says, and I quote:
If you don't have a balloon, you could potentially make this work from a mountain peak. Mount Thor, which you may remember from question #51, features a vertical drop of 1,250 meters. According to ballistics lab Close Focus Research, this is almost exactly how high a .22 Long Rifle bullet will fly if fired directly upward (...)
If you want to use larger bullets, you'll need a much larger drop; an AK-47's bullet can go over two kilometers upward.
The article is based on these figures:
So it all depends on what country you are on. If you live in a place like most urbanized regions of Canada, or Japan, where gun owners are rare and will usually be content with a pistol, you would be quite safe between 750 to 1250 meters above ground.
In some other places, such as...
... You might need to go way above three thousand meters above ground to be safe. And that's counting only the most assault rifle these days. I am no gun expert but I think weapons more suited for sniping could go higher.
Last but not least, guns are the least of your worries. Larry Walters flew his lawnchair over the primary approach corridor of the Los Angeles International Airport, which could have caused the first ever midair roadkilling in history. Given the current state of the world, were someone to do something like that nowadays, they would be taken down by missiles. Those can go much higher than bullets.
While several answers have done the calculations for rifle calibre firearms, we are actually missing a lot of important information. Are the people shooting at the blimp/aerostat firing as individuals or as formed bodies of troops? are they firing standard ammunition or have they gotten access to special ammunition like tracer, armour piercing and so on? Are they firing bolt action rifles, semi automatic rifles, fully automatic rifles (like a BAR) or using rifle calibre machine guns?
All these factors will change the amount of damage which the target will receive. A few disgruntled people firing rifles at a passing airship will be somewhat frightening, and there is always the chance a lucky hit will damage an engine or kill or injure a crew member, but the likelihood of that happening (absent the shooter being a sniper) is very low.
Formed bodies of troops become much more dangerous. A British rifle platoon just prior to WWI was quite capable of accurately estimating distances and firing up to 10 aimed rounds a minute. This level of firepower was enough to convince the Germans that they were facing machine-gun fire during the Battle of Mons in 1914. Any target in range will receive a concentrated blast of firepower, and the likelihood of an aerostat receiving critical damage or killed and injured crew is much higher. A 30 man platoon would put 300 rounds into the target in a minute, and it is likely they could reload quickly enough to engage the target several times before it could get out of range. If we assume a window long enough for 3 engagements, the platoon put 900 rounds in the target, and the likelihood of damage to structural members, cut wires and large holes torn in the envelope is quite high. The target may stagger away and crash later, but is likely to crash due to the amount of damage.
Don't be in their line of sight
We can simply keep increasing the amount of damage by changing to more modern weapons (although man portable automatic weapons like the Chauchat, Lewis Gun and BAR were actually available in the WWI period). You will now encounter higher rates of fire, and most semi automatic and fully automatic weapons also fire from 20 to 30 round magazines as standard, although higher capacity magazines have been developed over the years as well (a semi automatic AR-15 or an automatic M-4 can be fitted with a 100 round "C-Mag"). A 30 man platoon firing M-4 automatic rifles and equipped with 10 X 30 round magazines could put 9000 rounds in the target in a matter of minutes, tearing huge holes in the envelope and causing considerable damage to the structure and crew.
The final example would be belt fed rifle calibre machine guns. These can fire at rates from 600-1200 RPM depending on the type of gun (WWII era MG-42's had an exceptionally high firing rate).
This one man has the firepower of an entire WWI rifle platoon
So the amount of fire has far more bearing on the problem than just the calibre and range of the weapon and ammunition. Weapons fired by organized formations of troops will compound the damage done to the target, and the more advanced the weapons and the greater the amount of fire which can be brought to bear, the worst off the target will be. Modern Infantry platoons have a selection of belt fed machine guns and automatic rifles, so any target in range will have a huge issue.
To make things easy, I'm going to make a few assumptions and use generalities rather than math, because I don't want to do math.
You speak of household calibers, so I am going to list some very common ammunition types:
.22 LR .308 Winchester 5.56mm Nato .223 Remington 7.62mm .30-06 .45 .357 .38 Special 9mm
We can discount several of these because they are pistol rounds. Pistols are not long distance weapons. They rely on a balance of speed and bullet mass to do damage, followed by bullet shape and composition to do additional damage. Some I listed are also small game or varmint rounds.
When you take out the Varmint .22 LR and the pistol rounds like the .45, 9mm, .357, .38 special you are left with 5 rounds that may be a threat.
Actually that isn't entirely true, as the .223 Remington and the 5.56mm are almost identical, as are the .308 Winchester and the 7.62mm. we can actually lump the .30-06 in here as it is so close in size and performance.
The .308 Winchester is said to have an effective range close to 1000 meters in the hands of a skilled hunter. The 5.56 mm is said to have an effective range out to 500 meters (anecdotal for both rounds) That is assuming firing at a ground based target. You are going to make us shoot UP though. That's going to reduce the range a lot. I'd say your balloons are relatively safe from ground fire even from the .308 when they reach about 500 meters up.
Remember, effective range is different from how high can the round go. The round may go as high as 3000 meters, but at it's peak it's not moving fast enough to cause problems. Also, as a bullet loses speed, it may start to tumble, which increases air resistance, which slows it down faster.