Seriously; that’d be a crime.
(Heta na teppou mo kazu uteba ataru;
“Even an inept gun, fired a number of times, will hit.”)
Even if someone is very bad at something, with enough tries they’ll produce strong results or success every now and then out of pure chance. Take enough shots, and one of them is bound to get lucky. Bear in mind that this is about statistics, where good results are produced by random chance from a high volume of attempts, rather than the change-averse trait of “even a stopped clock is right twice a day,” in which (for me, at least), someone holds to one belief or strategy that is usually wrong, and benefits when the situation matches it.
We begin with noun 下手 (heta), “unskilled,” with particle な (na) allowing it to act as an adverb and modify the noun 鉄砲 (teppou), “gun.” This is marked by emphatic particle も (mo), here acting like the English adverb “even.” It also subsumes an implied topic-marker は (wa), and the remainder of the phrase is the comment on this topic.
The comment begins with what looks like the noun 数 (kazu), “number,” but is actually acting as an adverb: “frequently,” “repeatedly.” This modifies the verb 撃つ (utsu), “to hit,” “to attack,” “to shoot (at),” which appears in perfective form with conditional suffix ば (ba), “when.” And the following conditional clause consists of the verb 当たる (ataru), “to hit,” “to be accurate,” “to go well,” etc. (among myriad other possible translations) in conclusive form.
撃てば may be contracted to 撃ちゃ (ucha), and/or the kanji 撃 may be replaced with homophone 打, without any change in meaning.
A variant saying asserts that 下手な鍛冶屋も一度は名剣 (heta na kajiya mo ichido wa meiken), “Even an unskilled smith will produce one high-quality sword.”
This phrase can be insulting (as you’d expect, given the use of 下手), so use with caution in conversation or public discourse.
A couple of my sources claim that this comes from an English saying, “He that shoots oft at last shall hit the mark.” To be honest, I’ve never heard this before and most search results seem to be for Japanese pages making the same assertion, so take this with a grain of salt.
(“Heta na teppou mo kazu uteba ataru darou to omotte, seccho wo suujuusha no shuppan gaisha ni okuttte mimashita.”)
[“Believing that even a terrible marksman will hit the target with enough shots, I tried sending that little thing I wrote to several dozen publishing companies.”]